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Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition

Cover of Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition by Robin Nixon Published by O'Reilly Media, Inc.
  1. Learning PHP, MySQL, JavaScript, and CSS
  2. Dedication
  3. Preface
    1. Audience
    2. Assumptions This Book Makes
    3. Organization of This Book
    4. Supporting Books
    5. Conventions Used in This Book
    6. Using Code Examples
    7. Safari® Books Online
    8. How to Contact Us
    9. Acknowledgments
  4. 1. Introduction to Dynamic Web Content
    1. HTTP and HTML: Berners-Lee’s Basics
      1. The Request/Response Procedure
    2. The Benefits of PHP, MySQL, JavaScript, and CSS
      1. Using PHP
      2. Using MySQL
      3. Using JavaScript
      4. Using CSS
    3. The Apache Web Server
    4. About Open Source
    5. Bringing It All Together
    6. Test Your Knowledge
  5. 2. Setting Up a Development Server
    1. What Is a WAMP, MAMP, or LAMP?
    2. Installing a WAMP on Windows
      1. Testing the Installation
      2. Alternative WAMPs
    3. Installing a MAMP on OS X
      1. Configuring MySQL
      2. Testing the Installation
    4. Installing a LAMP on Linux
    5. Working Remotely
      1. Logging In
      2. Using FTP
    6. Using a Program Editor
    7. Using an IDE
    8. Test Your Knowledge
  6. 3. Introduction to PHP
    1. Incorporating PHP Within HTML
      1. Calling the PHP Parser
    2. This Book’s Examples
    3. The Structure of PHP
      1. Using Comments
      2. Basic Syntax
      3. Understanding Variables
      4. Operators
      5. Variable Assignment
      6. Multiple-Line Commands
      7. Variable Typing
      8. Constants
      9. The Difference Between the echo and print Commands
      10. Functions
      11. Variable Scope
    4. Test Your Knowledge
  7. 4. Expressions and Control Flow in PHP
    1. Expressions
      1. Literals and Variables
    2. Operators
      1. Operator Precedence
      2. Associativity
      3. Relational Operators
    3. Conditionals
      1. The if Statement
      2. The else Statement
      3. The elseif Statement
      4. The switch Statement
      5. The ? Operator
    4. Looping
      1. while Loops
      2. do…while Loops
      3. for Loops
      4. Breaking Out of a Loop
      5. The continue Statement
    5. Implicit and Explicit Casting
    6. PHP Dynamic Linking
      1. Dynamic Linking in Action
    7. Test Your Knowledge
  8. 5. PHP Functions and Objects
    1. PHP Functions
      1. Defining a Function
      2. Returning a Value
      3. Returning an Array
      4. Passing by Reference
      5. Returning Global Variables
      6. Recap of Variable Scope
    2. Including and Requiring Files
      1. The include Statement
      2. Using include_once
      3. Using require and require_once
    3. PHP Version Compatibility
    4. PHP Objects
      1. Terminology
      2. Declaring a Class
      3. Creating an Object
      4. Accessing Objects
      5. Constructors
      6. Writing Methods
      7. Declaring Properties
      8. Declaring Constants
      9. Property and Method Scope in PHP 5
      10. Inheritance
    5. Test Your Knowledge
  9. 6. PHP Arrays
    1. Basic Access
      1. Numerically Indexed Arrays
      2. Associative Arrays
      3. Assignment Using the array Keyword
    2. The foreach...as Loop
    3. Multidimensional Arrays
    4. Using Array Functions
      1. is_array
      2. count
      3. sort
      4. shuffle
      5. explode
      6. extract
      7. compact
      8. reset
      9. end
    5. Test Your Knowledge
  10. 7. Practical PHP
    1. Using printf
      1. Precision Setting
      2. String Padding
      3. Using sprintf
    2. Date and Time Functions
      1. Date Constants
      2. Using checkdate
    3. File Handling
      1. Checking Whether a File Exists
      2. Creating a File
      3. Reading from Files
      4. Copying Files
      5. Moving a File
      6. Deleting a File
      7. Updating Files
      8. Locking Files for Multiple Accesses
      9. Reading an Entire File
      10. Uploading Files
    4. System Calls
    5. XHTML
      1. The Benefits of XHTML
      2. XHTML Versions
      3. What’s Different?
      4. HTML 4.01 Document Types
      5. The HTML5 Document Type
      6. XHTML 1.0 Document Types
      7. XHTML Validation
    6. Test Your Knowledge
  11. 8. Introduction to MySQL
    1. MySQL Basics
    2. Summary of Database Terms
    3. Accessing MySQL via the Command Line
      1. Starting the Command-Line Interface
      2. Using the Command-Line Interface
      3. MySQL Commands
      4. Data Types
    4. Indexes
      1. Creating an Index
      2. Querying a MySQL Database
      3. Joining Tables Together
      4. Using Logical Operators
    5. MySQL Functions
    6. Accessing MySQL via phpMyAdmin
      1. Windows Users
      2. OS X Users
      3. Linux Users
      4. Using phpMyAdmin
    7. Test Your Knowledge
  12. 9. Mastering MySQL
    1. Database Design
      1. Primary Keys: The Keys to Relational Databases
    2. Normalization
      1. First Normal Form
      2. Second Normal Form
      3. Third Normal Form
      4. When Not to Use Normalization
    3. Relationships
      1. One-to-One
      2. One-to-Many
      3. Many-to-Many
      4. Databases and Anonymity
    4. Transactions
      1. Transaction Storage Engines
      2. Using BEGIN
      3. Using COMMIT
      4. Using ROLLBACK
    5. Using EXPLAIN
    6. Backing Up and Restoring
      1. Using mysqldump
      2. Creating a Backup File
      3. Restoring from a Backup File
      4. Dumping Data in CSV Format
      5. Planning Your Backups
    7. Test Your Knowledge
  13. 10. Accessing MySQL Using PHP
    1. Querying a MySQL Database with PHP
      1. The Process
      2. Creating a Login File
      3. Connecting to MySQL
    2. A Practical Example
      1. The $_POST Array
      2. Deleting a Record
      3. Displaying the Form
      4. Querying the Database
      5. Running the Program
    3. Practical MySQL
      1. Creating a Table
      2. Describing a Table
      3. Dropping a Table
      4. Adding Data
      5. Retrieving Data
      6. Updating Data
      7. Deleting Data
      8. Using AUTO_INCREMENT
      9. Performing Additional Queries
      10. Preventing SQL Injection
      11. Preventing HTML Injection
    4. Test Your Knowledge
  14. 11. Form Handling
    1. Building Forms
    2. Retrieving Submitted Data
      1. register_globals: An Old Solution Hangs On
      2. Default Values
      3. Input Types
      4. Sanitizing Input
    3. An Example Program
    4. Test Your Knowledge
  15. 12. Cookies, Sessions, and Authentication
    1. Using Cookies in PHP
      1. Setting a Cookie
      2. Accessing a Cookie
      3. Destroying a Cookie
    2. HTTP Authentication
      1. Storing Usernames and Passwords
      2. Salting
    3. Using Sessions
      1. Starting a Session
      2. Ending a Session
      3. Session Security
    4. Test Your Knowledge
  16. 13. Exploring JavaScript
    1. JavaScript and HTML Text
      1. Using Scripts Within a Document Head
      2. Older and Nonstandard Browsers
      3. Including JavaScript Files
      4. Debugging JavaScript Errors
    2. Using Comments
    3. Semicolons
    4. Variables
      1. String Variables
      2. Numeric Variables
      3. Arrays
    5. Operators
      1. Arithmetic Operators
      2. Assignment Operators
      3. Comparison Operators
      4. Logical Operators
      5. Variable Incrementing and Decrementing
      6. String Concatenation
      7. Escaping Characters
    6. Variable Typing
    7. Functions
    8. Global Variables
      1. Local Variables
    9. The Document Object Model (DOM)
      1. But It’s Not That Simple
      2. Using the DOM
    10. Test Your Knowledge
  17. 14. Expressions and Control Flow in JavaScript
    1. Expressions
      1. Literals and Variables
    2. Operators
      1. Operator Precedence
      2. Associativity
      3. Relational Operators
    3. The with Statement
    4. Using onerror
    5. Using try...catch
    6. Conditionals
      1. The if Statement
      2. The switch statement
      3. The ? Operator
    7. Looping
      1. while Loops
      2. do…while Loops
      3. for Loops
      4. Breaking Out of a Loop
      5. The continue Statement
    8. Explicit Casting
    9. Test Your Knowledge
  18. 15. JavaScript Functions, Objects, and Arrays
    1. JavaScript Functions
      1. Defining a Function
      2. Returning a Value
      3. Returning an Array
    2. JavaScript Objects
      1. Declaring a Class
      2. Creating an Object
      3. Accessing Objects
      4. The prototype Keyword
    3. JavaScript Arrays
      1. Numeric Arrays
      2. Associative Arrays
      3. Multidimensional Arrays
      4. Using Array Methods
    4. Test Your Knowledge
  19. 16. JavaScript and PHP Validation and Error Handling
    1. Validating User Input with JavaScript
      1. The validate.html Document (Part One)
      2. The validate.html Document (Part Two)
    2. Regular Expressions
      1. Matching Through Metacharacters
      2. Fuzzy Character Matching
      3. Grouping Through Parentheses
      4. Character Classes
      5. Some More Complicated Examples
      6. Summary of Metacharacters
      7. General Modifiers
      8. Using Regular Expressions in JavaScript
      9. Using Regular Expressions in PHP
    3. Redisplaying a Form After PHP Validation
    4. Test Your Knowledge
  20. 17. Using Ajax
    1. What Is Ajax?
    2. Using XMLHttpRequest
    3. Implementing Ajax via POST Requests
      1. The readyState Property
      2. The Server Half of the Ajax Process
    4. Using GET Instead of POST
    5. Sending XML Requests
      1. About XML
      2. Why Use XML?
    6. Using Frameworks for Ajax
    7. Test Your Knowledge
  21. 18. Introduction to CSS
    1. Importing a Style Sheet
      1. Importing a Style Sheet from Within HTML
    2. Embedded Style Settings
      1. Using IDs
      2. Using Classes
    3. CSS Rules
      1. Using Semicolons
      2. Multiple Assignments
      3. Using Comments
    4. Style Types
      1. Default Styles
      2. User Styles
      3. External Style Sheets
      4. Internal Styles
      5. Inline Styles
    5. CSS Selectors
      1. The Type Selector
      2. The Descendant Selector
      3. The Child Selector
      4. The Adjacent Sibling Selector
      5. The ID Selector
      6. The Class Selector
      7. The Attribute Selector
      8. The Universal Selector
      9. Selecting by Group
    6. The CSS Cascade
      1. Style Sheet Creators
      2. Style Sheet Methods
      3. Style Sheet Selectors
    7. The Difference Between <div> and <span>
    8. Measurements
    9. Fonts and Typography
      1. font-family
      2. font-style
      3. font-size
      4. font-weight
    10. Managing Text Styles
      1. Decoration
      2. Spacing
      3. Alignment
      4. Transformation
      5. Indenting
    11. CSS Colors
      1. Short Color Strings
      2. Gradients
    12. Positioning Elements
      1. Absolute Positioning
      2. Relative Positioning
      3. Fixed Positioning
      4. Comparing Positioning Types
    13. Pseudoclasses
    14. Pseudoelements
    15. Shorthand Rules
    16. The Box Model and Layout
      1. Setting Margins
      2. Applying Borders
      3. Adjusting Padding
      4. Object Contents
    17. Test Your Knowledge
  22. 19. Advanced CSS with CSS3
    1. Attribute Selectors
      1. Matching Parts of Strings
    2. The box-sizing Property
    3. CSS3 Backgrounds
      1. The background-clip Property
      2. The background-origin Property
      3. The background-size Property
      4. Multiple Backgrounds
    4. CSS3 Borders
      1. The border-color Property
      2. The border-radius Property
    5. Box Shadows
    6. Element Overflow
    7. Multicolumn Layout
    8. Colors and Opacity
      1. HSL Colors
      2. HSLA Colors
      3. RGB Colors
      4. RGBA Colors
      5. The opacity Property
    9. Text Effects
      1. The text-shadow Property
      2. The text-overflow Property
      3. The word-wrap Property
    10. Web Fonts
      1. Google Web Fonts
    11. Transformations
    12. Transitions
      1. Properties to Transition
      2. Transition Duration
      3. Transition Delay
      4. Transition Timing
      5. Shorthand Syntax
    13. Test Your Knowledge
  23. 20. Accessing CSS from JavaScript
    1. Revisiting the getElementById Function
      1. The O Function
      2. The S Function
      3. The C Function
      4. Including the Functions
    2. Accessing CSS Properties from JavaScript
      1. Some Common Properties
      2. Other Properties
    3. Inline JavaScript
      1. The this Keyword
      2. Attaching Events to Objects in a Script
      3. Attaching to Other Events
    4. Adding New Elements
      1. Removing Elements
      2. Alternatives to Adding and Removing Elements
    5. Using Interrupts
      1. Using setTimeout
      2. Canceling a Timeout
      3. Using setInterval
      4. Using Interrupts for Animation
    6. Test Your Knowledge
  24. 21. Bringing It All Together
    1. Designing a Social Networking Site
    2. On the Website
    3. functions.php
      1. The Functions
    4. header.php
    5. setup.php
    6. index.php
    7. signup.php
      1. Checking for Username Availability
    8. checkuser.php
    9. login.php
    10. profile.php
      1. Adding the “About Me” Text
      2. Adding a Profile Image
      3. Processing the Image
      4. Displaying the Current Profile
    11. members.php
      1. Viewing a User’s Profile
      2. Adding and Dropping Friends
      3. Listing All Members
    12. friends.php
    13. messages.php
    14. logout.php
    15. styles.css
  25. A. Solutions to the Chapter Questions
    1. Chapter 1 Answers
    2. Chapter 2 Answers
    3. Chapter 3 Answers
    4. Chapter 4 Answers
    5. Chapter 5 Answers
    6. Chapter 6 Answers
    7. Chapter 7 Answers
    8. Chapter 8 Answers
    9. Chapter 9 Answers
    10. Chapter 10 Answers
    11. Chapter 11 Answers
    12. Chapter 12 Answers
    13. Chapter 13 Answers
    14. Chapter 14 Answers
    15. Chapter 15 Answers
    16. Chapter 16 Answers
    17. Chapter 17 Answers
    18. Chapter 18 Answers
    19. Chapter 19 Answers
    20. Chapter 20 Answers
  26. B. Online Resources
    1. PHP Resource Sites
    2. MySQL Resource Sites
    3. JavaScript Resource Sites
    4. Ajax Resource Sites
    5. Miscellaneous Resource Sites
    6. O’Reilly Resource Sites
  27. C. MySQL’s FULLTEXT Stopwords
  28. D. MySQL Functions
    1. String Functions
      1. CONCAT()
      2. CONCAT_WS()
      3. LEFT()
      4. RIGHT()
      5. MID()
      6. LENGTH()
      7. LPAD()
      8. RPAD
      9. LOCATE()
      10. LOWER()
      11. UPPER()
      12. QUOTE()
      13. REPEAT()
      14. REPLACE()
      15. TRIM()
      16. LTRIM() and RTRIM()
    2. Date Functions
      1. CURDATE()
      2. DATE()
      3. DATE_ADD()
      4. DATE_FORMAT()
      5. DAY()
      6. DAYNAME()
      7. DAYOFWEEK()
      8. DAYOFYEAR()
      9. LAST_DAY()
      10. MAKEDATE()
      11. MONTH()
      12. MONTHNAME()
      13. SYSDATE()
      14. YEAR()
      15. WEEK()
      16. WEEKDAY()
    3. Time Functions
      1. CURTIME()
      2. HOUR()
      3. MINUTE()
      4. SECOND()
      5. MAKETIME()
      6. TIMEDIFF()
      7. UNIX_TIMESTAMP()
      8. FROM_UNIXTIME()
  29. Index
  30. About the Author
  31. Colophon
  32. Copyright
O'Reilly logo

Conditionals

Conditionals alter program flow. They enable you to ask questions about certain things and respond to the answers you get in different ways. Conditionals are central to dynamic web pages—the goal of using PHP in the first place—because they make it easy to create different output each time a page is viewed.

There are three types of nonlooping conditionals: the if statement, the switch statement, and the ? operator. By nonlooping, I mean that the actions initiated by the statement take place and program flow then moves on, whereas looping conditionals (which we’ll come to shortly) execute code over and over until a condition has been met.

The if Statement

One way of thinking about program flow is to imagine it as a single-lane highway that you are driving along. It’s pretty much a straight line, but now and then you encounter various signs telling you where to go.

In the case of an if statement, you could imagine coming across a detour sign that you have to follow if a certain condition is TRUE. If so, you drive off and follow the detour until you rejoin your original route; you then continue on your way in your original direction. Or, if the condition isn’t TRUE, you ignore the detour and carry on driving (see Figure 4-1).

Program flow is like a single-lane highway

Figure 4-1. Program flow is like a single-lane highway

The contents of the if condition can be any valid PHP expression, including equality, comparison, tests for zero and NULL, and even the values returned by functions (either built-in functions or ones that you write).

The action to take when an if condition is TRUE are generally placed inside curly braces, {}. You can omit the braces if you have only a single statement to execute, but if you always use curly braces you’ll avoid potentially difficult-to-trace bugs, such as when you add an extra line to a condition but forget to add the braces in, so it doesn’t get evaluated. (Note that for reasons of layout and clarity, many of the examples in this book ignore this suggestion and omit the braces for single statements.)

In Example 4-19, imagine that it is the end of the month and all your bills have been paid, so you are performing some bank account maintenance.

Example 4-19. An if statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
?>

In this example, you are checking your balance to see whether it is less than 100 dollars (or whatever your currency is). If so, you pay yourself 1000 dollars and then add it to the balance. (If only making money were that simple!)

If the bank balance is 100 dollars or greater, the conditional statements are ignored and program flow skips to the next line (not shown).

In this book, opening curly braces generally start on a new line. Some people like to place the first curly brace to the right of the conditional expression instead. Either of these approaches is fine, because PHP allows you to set out your whitespace characters (spaces, newlines, and tabs) any way you choose. However, you will find your code easier to read and debug if you indent each level of conditionals with a tab.

The else Statement

Sometimes when a conditional is not TRUE, you may not want to continue on to the main program code immediately but might wish to do something else instead. This is where the else statement comes in. With it, you can set up a second detour on your highway, as in Figure 4-2.

The highway now has an if detour and an else detour

Figure 4-2. The highway now has an if detour and an else detour

What happens with an if...else statement is that the first conditional statement is executed if the condition is TRUE, but if it’s FALSE, the second one is executed. One of the two choices must be executed. Under no circumstances can both (or neither) be executed. Example 4-20 shows the use of the if...else structure.

Example 4-20. An if…else statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
else
{
    $savings      += 50;
    $bank_balance -= 50;
}
?>

In this example, having ascertained that you have over $100 in the bank, the else statement is executed, by which you place some of this money into your savings account.

As with if statements, if your else has only one conditional statement, you can opt to leave out the curly braces. (Curly braces are always recommended, though: they make the code easier to understand, and they let you easily add more statements to the branch later.)

The elseif Statement

There are also times when you want a number of different possibilities to occur, based upon a sequence of conditions. You can achieve this using the elseif statement. As you might imagine, it is like an else statement, except that you place a further conditional expression prior to the conditional code. In Example 4-21, you can see a complete if...elseif...else construct.

Example 4-21. An if…elseif…else statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
elseif ($bank_balance > 200)
{
    $savings      += 100;
    $bank_balance -= 100;
}
else
{
    $savings      += 50;
    $bank_balance -= 50;
}
?>

In this example, an elseif statement has been inserted between the if and else statements. It checks whether your bank balance exceeds $200 and, if so, decides that you can afford to save $100 of it this month.

Although I’m starting to stretch the metaphor a bit too far, you can imagine this as a multiway set of detours (see Figure 4-3).

The highway with if, elseif, and else detours

Figure 4-3. The highway with if, elseif, and else detours

Note

An else statement closes one of the following: an if...else statement or an if...elseif...else statement. You can leave out a final else if it is not required, but you cannot have one before an elseif; neither can you have an elseif before an if statement.

You may have as many elseif statements as you like, but as the number of elseif statements increases it becomes advisable to consider a switch statement instead, if it fits your needs. We’ll look at that next.

The switch Statement

The switch statement is useful in cases in which one variable or the result of an expression can have multiple values, which should each trigger a different function.

For example, consider a PHP-driven menu system that passes a single string to the main menu code according to what the user requests. Let’s say the options are Home, About, News, Login, and Links, and we set the variable $page to one of these, according to the user’s input.

The code for this written using if...elseif...else might look like Example 4-22.

Example 4-22. A multiple-line if…elseif…statement

<?php
if     ($page == "Home")  echo "You selected Home";
elseif ($page == "About") echo "You selected About";
elseif ($page == "News")  echo "You selected News";
elseif ($page == "Login") echo "You selected Login";
elseif ($page == "Links") echo "You selected Links";
?>

Using a switch statement, the code might look like Example 4-23.

Example 4-23. A switch statement

<?php
switch ($page)
{
    case "Home":
        echo "You selected Home";
        break;
    case "About":
        echo "You selected About";
        break;
    case "News":
        echo "You selected News";
        break;
    case "Login":
        echo "You selected Login";
        break;
    case "Links":
        echo "You selected Links";
        break;
}
?>

As you can see, $page is mentioned only once at the start of the switch statement. Thereafter, the case command checks for matches. When one occurs, the matching conditional statement is executed. Of course, in a real program you would have code here to display or jump to a page, rather than simply telling the user what was selected.

Note

One thing to note about switch statements is that you do not use curly braces inside case commands. Instead, they commence with a colon and end with the break statement. The entire list of cases in the switch statement is enclosed in a set of curly braces, though.

Breaking out

If you wish to break out of the switch statement because a condition has been fulfilled, use the break command. This command tells PHP to break out of the switch and jump to the following statement.

If you were to leave out the break commands in Example 4-23 and the case of “Home” evaluated to be TRUE, all five cases would then be executed. Or if $page had the value “News,” all the case commands from then on would execute. This is deliberate and allows for some advanced programming, but generally you should always remember to issue a break command every time a set of case conditionals has finished executing. In fact, leaving out the break statement is a common error.

Default action

A typical requirement in switch statements is to fall back on a default action if none of the case conditions are met. For example, in the case of the menu code in Example 4-23, you could add the code in Example 4-24 immediately before the final curly brace.

Example 4-24. A default statement to add to Example 4-23

    default: echo "Unrecognized selection";
        break;

Although a break command is not required here because the default is the final substatement, and program flow will automatically continue to the closing curly brace, should you decide to place the default statement higher up it would definitely need a break command to prevent program flow from dropping into the following statements. Generally, the safest practice is to always include the break command.

Alternative syntax

If you prefer, you may replace the first curly brace in a switch statement with a single colon and the final curly brace with an endswitch command, as in Example 4-25. However, this approach is not commonly used and is mentioned here only in case you encounter it in third-party code.

Example 4-25. Alternate switch statement syntax

<?php
switch ($page):
    case "Home":
        echo "You selected Home";
        break;

    // etc...

    case "Links":
        echo "You selected Links";
        break;
endswitch;
?>

The ? Operator

One way of avoiding the verbosity of if and else statements is to use the more compact ternary operator, ?, which is unusual in that it takes three operands rather than the more usual two.

We briefly came across this in Chapter 3 in the discussion about the difference between the print and echo statements, as an example of an operator type that works well with print but not echo.

The ? operator is passed an expression that it must evaluate, along with two statements to execute: one for when the expression evaluates to TRUE, the other for when it is FALSE. Example 4-26 shows code we might use for writing a warning about the fuel level of a car to its digital dashboard.

Example 4-26. Using the ? operator

<?php
echo $fuel <= 1 ? "Fill tank now" : "There's enough fuel";
?>

In this statement, if there is one gallon or less of fuel (in other words, if $fuel is set to 1 or less), the string “Fill tank now” is returned to the preceding echo statement. Otherwise, the string “There’s enough fuel” is returned. You can also assign the value returned in a ? statement to a variable (see Example 4-27).

Example 4-27. Assigning a ? conditional result to a variable

<?php
$enough = $fuel <= 1 ? FALSE : TRUE;
?>

Here, $enough will be assigned the value TRUE only when there is more than a gallon of fuel; otherwise, it is assigned the value FALSE.

If you find the ? operator confusing, you are free to stick to if statements, but you should be familiar with it because you’ll see it in other people’s code. It can be hard to read, because it often mixes multiple occurrences of the same variable. For instance, code such as the following is quite popular:

$saved = $saved >= $new ? $saved : $new;

If you take it apart carefully, you can figure out what this code does:

$saved =                    // Set the value of $saved
        $saved >= $new      // Check $saved against $new
    ?                       // Yes, comparison is true ...
        $saved              // ... so assign the current value of $saved
    :                       // No, comparison is false ...
        $new;               // ... so assign the value of $new

It’s a concise way to keep track of the largest value that you’ve seen as a program progresses. You save the largest value in $saved and compare it to $new each time you get a new value. Programmers familiar with the ? operator find it more convenient than if statements for such short comparisons. When not used for writing compact code, it is typically used to make some decision inline, such as when testing whether a variable is set before passing it to a function.

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