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Programming PHP, 3rd Edition

Cover of Programming PHP, 3rd Edition by Kevin Tatroe... Published by O'Reilly Media, Inc.
  1. Dedication
  2. Special Upgrade Offer
  3. Foreword
  4. Preface
    1. Audience
    2. Assumptions This Book Makes
    3. Contents of This Book
    4. Conventions Used in This Book
    5. Using Code Examples
    6. Safari® Books Online
    7. How to Contact Us
    8. Acknowledgments
      1. Kevin Tatroe
      2. Peter MacIntyre
  5. 1. Introduction to PHP
    1. What Does PHP Do?
    2. A Brief History of PHP
      1. The Evolution of PHP
      2. The Widespread Use of PHP
    3. Installing PHP
    4. A Walk Through PHP
      1. Configuration Page
      2. Forms
      3. Databases
      4. Graphics
  6. 2. Language Basics
    1. Lexical Structure
      1. Case Sensitivity
      2. Statements and Semicolons
      3. Whitespace and Line Breaks
      4. Comments
      5. Literals
      6. Identifiers
      7. Keywords
    2. Data Types
      1. Integers
      2. Floating-Point Numbers
      3. Strings
      4. Booleans
      5. Arrays
      6. Objects
      7. Resources
      8. Callbacks
      9. NULL
    3. Variables
      1. Variable Variables
      2. Variable References
      3. Variable Scope
      4. Garbage Collection
    4. Expressions and Operators
      1. Number of Operands
      2. Operator Precedence
      3. Operator Associativity
      4. Implicit Casting
      5. Arithmetic Operators
      6. String Concatenation Operator
      7. Auto-increment and Auto-decrement Operators
      8. Comparison Operators
      9. Bitwise Operators
      10. Logical Operators
      11. Casting Operators
      12. Assignment Operators
      13. Miscellaneous Operators
    5. Flow-Control Statements
      1. if
      2. switch
      3. while
      4. for
      5. foreach
      6. try...catch
      7. declare
      8. exit and return
      9. goto
    6. Including Code
    7. Embedding PHP in Web Pages
      1. Standard (XML) Style
      2. SGML Style
      3. ASP Style
      4. Script Style
      5. Echoing Content Directly
  7. 3. Functions
    1. Calling a Function
    2. Defining a Function
    3. Variable Scope
      1. Global Variables
      2. Static Variables
    4. Function Parameters
      1. Passing Parameters by Value
      2. Passing Parameters by Reference
      3. Default Parameters
      4. Variable Parameters
      5. Missing Parameters
      6. Type Hinting
    5. Return Values
    6. Variable Functions
    7. Anonymous Functions
  8. 4. Strings
    1. Quoting String Constants
      1. Variable Interpolation
      2. Single-Quoted Strings
      3. Double-Quoted Strings
      4. Here Documents
    2. Printing Strings
      1. echo
      2. print()
      3. printf()
      4. print_r() and var_dump()
    3. Accessing Individual Characters
    4. Cleaning Strings
      1. Removing Whitespace
      2. Changing Case
    5. Encoding and Escaping
      1. HTML
      2. URLs
      3. SQL
      4. C-String Encoding
    6. Comparing Strings
      1. Exact Comparisons
      2. Approximate Equality
    7. Manipulating and Searching Strings
      1. Substrings
      2. Miscellaneous String Functions
      3. Decomposing a String
      4. String-Searching Functions
    8. Regular Expressions
      1. The Basics
      2. Character Classes
      3. Alternatives
      4. Repeating Sequences
      5. Subpatterns
      6. Delimiters
      7. Match Behavior
      8. Character Classes
      9. Anchors
      10. Quantifiers and Greed
      11. Noncapturing Groups
      12. Backreferences
      13. Trailing Options
      14. Inline Options
      15. Lookahead and Lookbehind
      16. Cut
      17. Conditional Expressions
      18. Functions
      19. Differences from Perl Regular Expressions
  9. 5. Arrays
    1. Indexed Versus Associative Arrays
    2. Identifying Elements of an Array
    3. Storing Data in Arrays
      1. Adding Values to the End of an Array
      2. Assigning a Range of Values
      3. Getting the Size of an Array
      4. Padding an Array
    4. Multidimensional Arrays
    5. Extracting Multiple Values
      1. Slicing an Array
      2. Splitting an Array into Chunks
      3. Keys and Values
      4. Checking Whether an Element Exists
      5. Removing and Inserting Elements in an Array
    6. Converting Between Arrays and Variables
      1. Creating Variables from an Array
      2. Creating an Array from Variables
    7. Traversing Arrays
      1. The foreach Construct
      2. The Iterator Functions
      3. Using a for Loop
      4. Calling a Function for Each Array Element
      5. Reducing an Array
      6. Searching for Values
    8. Sorting
      1. Sorting One Array at a Time
      2. Natural-Order Sorting
      3. Sorting Multiple Arrays at Once
      4. Reversing Arrays
      5. Randomizing Order
    9. Acting on Entire Arrays
      1. Calculating the Sum of an Array
      2. Merging Two Arrays
      3. Calculating the Difference Between Two Arrays
      4. Filtering Elements from an Array
    10. Using Arrays
      1. Sets
      2. Stacks
    11. Iterator Interface
  10. 6. Objects
    1. Terminology
    2. Creating an Object
    3. Accessing Properties and Methods
    4. Declaring a Class
      1. Declaring Methods
      2. Declaring Properties
      3. Declaring Constants
      4. Inheritance
      5. Interfaces
      6. Traits
      7. Abstract Methods
      8. Constructors
      9. Destructors
    5. Introspection
      1. Examining Classes
      2. Examining an Object
      3. Sample Introspection Program
    6. Serialization
  11. 7. Web Techniques
    1. HTTP Basics
    2. Variables
    3. Server Information
    4. Processing Forms
      1. Methods
      2. Parameters
      3. Self-Processing Pages
      4. Sticky Forms
      5. Multivalued Parameters
      6. Sticky Multivalued Parameters
      7. File Uploads
      8. Form Validation
    5. Setting Response Headers
      1. Different Content Types
      2. Redirections
      3. Expiration
      4. Authentication
    6. Maintaining State
      1. Cookies
      2. Sessions
      3. Combining Cookies and Sessions
    7. SSL
  12. 8. Databases
    1. Using PHP to Access a Database
    2. Relational Databases and SQL
      1. PHP Data Objects
    3. MySQLi Object Interface
      1. Retrieving Data for Display
    4. SQLite
    5. Direct File-Level Manipulation
    6. MongoDB
      1. Retrieving Data
      2. Inserting More Complex Data
  13. 9. Graphics
    1. Embedding an Image in a Page
    2. Basic Graphics Concepts
    3. Creating and Drawing Images
      1. The Structure of a Graphics Program
      2. Changing the Output Format
      3. Testing for Supported Image Formats
      4. Reading an Existing File
      5. Basic Drawing Functions
    4. Images with Text
      1. Fonts
      2. TrueType Fonts
    5. Dynamically Generated Buttons
      1. Caching the Dynamically Generated Buttons
      2. A Faster Cache
    6. Scaling Images
    7. Color Handling
      1. Using the Alpha Channel
      2. Identifying Colors
      3. True Color Indexes
      4. Text Representation of an Image
  14. 10. PDF
    1. PDF Extensions
    2. Documents and Pages
      1. A Simple Example
      2. Initializing the Document
      3. Outputting Basic Text Cells
    3. Text
      1. Coordinates
      2. Text Attributes
      3. Page Headers, Footers, and Class Extension
      4. Images and Links
      5. Tables and Data
  15. 11. XML
    1. Lightning Guide to XML
    2. Generating XML
    3. Parsing XML
      1. Element Handlers
      2. Character Data Handler
      3. Processing Instructions
      4. Entity Handlers
      5. Default Handler
      6. Options
      7. Using the Parser
      8. Errors
      9. Methods as Handlers
      10. Sample Parsing Application
    4. Parsing XML with DOM
    5. Parsing XML with SimpleXML
    6. Transforming XML with XSLT
  16. 12. Security
    1. Filter Input
    2. Cross-Site Scripting
      1. SQL Injection
    3. Escape Output
      1. Filenames
    4. Session Fixation
    5. File Uploads
      1. Distrust Browser-Supplied Filenames
      2. Beware of Filling Your Filesystem
      3. Surviving register_globals
    6. File Access
      1. Restrict Filesystem Access to a Specific Directory
      2. Get It Right the First Time
      3. Don’t Use Files
      4. Session Files
      5. Concealing PHP Libraries
    7. PHP Code
    8. Shell Commands
    9. More Information
    10. Security Recap
  17. 13. Application Techniques
    1. Code Libraries
    2. Templating Systems
    3. Handling Output
      1. Output Buffering
      2. Compressing Output
    4. Error Handling
      1. Error Reporting
      2. Error Suppression
      3. Triggering Errors
      4. Defining Error Handlers
    5. Performance Tuning
      1. Benchmarking
      2. Profiling
      3. Optimizing Execution Time
      4. Optimizing Memory Requirements
      5. Reverse Proxies and Replication
  18. 14. PHP on Disparate Platforms
    1. Writing Portable Code for Windows and Unix
      1. Determining the Platform
      2. Handling Paths Across Platforms
      3. The Server Environment
      4. Sending Mail
      5. End-of-Line Handling
      6. End-of-File Handling
      7. External Commands
      8. Common Platform-Specific Extensions
    2. Interfacing with COM
      1. Background
      2. PHP Functions
      3. Determining the API
  19. 15. Web Services
    1. REST Clients
      1. Responses
      2. Retrieving Resources
      3. Updating Resources
      4. Creating Resources
      5. Deleting Resources
    2. XML-RPC
      1. Servers
      2. Clients
  20. 16. Debugging PHP
    1. The Development Environment
    2. The Staging Environment
    3. The Production Environment
    4. php.ini Settings
    5. Manual Debugging
    6. Error Log
    7. IDE Debugging
    8. Additional Debugging Techniques
  21. 17. Dates and Times
  22. A. Function Reference
    1. PHP Functions by Category
      1. Arrays
      2. Classes and Objects
      3. Date and Time
      4. Directories
      5. Errors and Logging
      6. Program Execution
      7. Filesystem
      8. Data Filtering
      9. Functions
      10. PHP Options/Info
      11. Mail
      12. Math
      13. Miscellaneous Functions
      14. Network
      15. Output Buffering
      16. Session Handling
      17. Streams
      18. Strings
      19. PHP Language Tokenizer
      20. URLs
      21. Variables
    2. Alphabetical Listing of PHP Functions
  23. Index
  24. About the Authors
  25. Colophon
  26. Special Upgrade Offer
  27. Copyright
O'Reilly logo

Comparing Strings

PHP has two operators and six functions for comparing strings to each other.

Exact Comparisons

You can compare two strings for equality with the == and === operators. These operators differ in how they deal with nonstring operands. The == operator casts nonstring operands to strings, so it reports that 3 and "3" are equal. The === operator does not cast, and returns false if the data types of the arguments differ:

$o1 = 3;
$o2 = "3";

if ($o1 == $o2) {
  echo("== returns true<br>");
}
if ($o1 === $o2) {
  echo("=== returns true<br>");
}
== returns true

The comparison operators (<, <=, >, >=) also work on strings:

$him = "Fred";
$her = "Wilma";

if ($him < $her) {
  print "{$him} comes before {$her} in the alphabet.\n";
}
Fred comes before Wilma in the alphabet

However, the comparison operators give unexpected results when comparing strings and numbers:

$string = "PHP Rocks";
$number = 5;

if ($string < $number) {
  echo("{$string} < {$number}");
}
PHP Rocks < 5

When one argument to a comparison operator is a number, the other argument is cast to a number. This means that "PHP Rocks" is cast to a number, giving 0 (since the string does not start with a number). Because 0 is less than 5, PHP prints "PHP Rocks < 5".

To explicitly compare two strings as strings, casting numbers to strings if necessary, use the strcmp() function:

$relationship = strcmp(string_1, string_2);

The function returns a number less than 0 if string_1 sorts before string_2, greater than 0 if string_2 sorts before string_1, or 0 if they are the same:

$n = strcmp("PHP Rocks", 5);
echo($n);
1

A variation on strcmp() is strcasecmp(), which converts strings to lowercase before comparing them. Its arguments and return values are the same as those for strcmp():

$n = strcasecmp("Fred", "frED");   // $n is 0

Another variation on string comparison is to compare only the first few characters of the string. The strncmp() and strncasecmp() functions take an additional argument, the initial number of characters to use for the comparisons:

$relationship = strncmp(string_1, string_2, len);
$relationship = strncasecmp(string_1, string_2, len);

The final variation on these functions is natural-order comparison with strnatcmp() and strnatcasecmp(), which take the same arguments as strcmp() and return the same kinds of values. Natural-order comparison identifies numeric portions of the strings being compared and sorts the string parts separately from the numeric parts.

Table 4-5 shows strings in natural order and ASCII order.

Table 4-5. Natural order versus ASCII order

Natural order

ASCII order

pic1.jpg

pic1.jpg

pic5.jpg

pic10.jpg

pic10.jpg

pic5.jpg

pic50.jpg

pic50.jpg

Approximate Equality

PHP provides several functions that let you test whether two strings are approximately equal: soundex(), metaphone(), similar_text(), and levenshtein():

$soundexCode = soundex($string);
$metaphoneCode = metaphone($string);
$inCommon = similar_text($string_1, $string_2 [, $percentage ]);
$similarity = levenshtein($string_1, $string_2);
$similarity = levenshtein($string_1, $string_2 [, $cost_ins, $cost_rep, $cost_del ]);

The Soundex and Metaphone algorithms each yield a string that represents roughly how a word is pronounced in English. To see whether two strings are approximately equal with these algorithms, compare their pronunciations. You can compare Soundex values only to Soundex values and Metaphone values only to Metaphone values. The Metaphone algorithm is generally more accurate, as the following example demonstrates:

$known = "Fred";
$query = "Phred";

if (soundex($known) == soundex($query)) {
  print "soundex: {$known} sounds like {$query}<br>";
}
else {
  print "soundex: {$known} doesn't sound like {$query}<br>";
}

if (metaphone($known) == metaphone($query)) {
  print "metaphone: {$known} sounds like {$query}<br>";
}
else {
  print "metaphone: {$known} doesn't sound like {$query}<br>";
}
soundex: Fred doesn't sound like Phred
metaphone: Fred sounds like Phred

The similar_text() function returns the number of characters that its two string arguments have in common. The third argument, if present, is a variable in which to store the commonality as a percentage:

$string1 = "Rasmus Lerdorf";
$string2 = "Razmus Lehrdorf";
$common = similar_text($string1, $string2, $percent);
printf("They have %d chars in common (%.2f%%).", $common, $percent);
They have 13 chars in common (89.66%).

The Levenshtein algorithm calculates the similarity of two strings based on how many characters you must add, substitute, or remove to make them the same. For instance, "cat" and "cot" have a Levenshtein distance of 1, because you need to change only one character (the "a" to an "o") to make them the same:

$similarity = levenshtein("cat", "cot"); // $similarity is 1

This measure of similarity is generally quicker to calculate than that used by the similar_text() function. Optionally, you can pass three values to the levenshtein() function to individually weight insertions, deletions, and replacements—for instance, to compare a word against a contraction.

This example excessively weights insertions when comparing a string against its possible contraction, because contractions should never insert characters:

echo levenshtein('would not', 'wouldn\'t', 500, 1, 1);

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