This section lays out some ground rules about the structure of PHP programs. More foundational than the basics such as "how do I print something" or "how do I add two numbers", these proto-basics are the equivalent of someone telling you that you should read pages in this book from top to bottom and left to right, or that what's important on the page are the black squiggles, not the large white areas.
If you've had a little experience with PHP already or you're the kind of person that prefers playing with all the buttons on your new DVD player before going back and reading in the manual about how the buttons actually work, feel free to skip ahead to Chapter 2 now and flip back here later. If you forge ahead to write some PHP programs of your own, and they're behaving unexpectedly or the PHP interpreter complains of "parse errors" when it tries to run your program, revisit this section for a refresher.
the examples you've already seen in this chapter uses
<?php as the PHP start tag and
?> as the PHP end tag. The PHP interpreter ignores anything outside of
those tags. Text before the start tag or after the end tag is printed with no
interference from the PHP interpreter.
A PHP program can have multiple start and end tag pairs, as shown in Example 1-8.
Example 1-8. Multiple start and end tags
Five plus five is: <?php print 5 + 5; ?> <p> Four plus four is: <?php print 4 + 4; ?> <p> <img src="vacation.jpg" alt="My Vacation">
The PHP source code inside each set of
?> tags is processed by the PHP interpreter,
and the rest of the page is printed as is. Example 1-8 prints:
Five plus five is: 10<p> Four plus four is: 8<p> <img src="vacation.jpg" alt="My Vacation">
Some older PHP programs use
<? as a
start tag instead of
<? is called the
short open tag, since it's shorter than
<?php. It's usually better to use the regular
<?php open tag since it's guaranteed to work on any server running
the PHP interpreter. The short tag can be turned on or off with a PHP configuration
setting. Appendix A shows you how to modify
your PHP configuration to control which open tags are valid in your programs.
The rest of the examples in this chapter all begin with the
<?php start tag and end with
?>. In subsequent chapters, not all the examples have start and end
tags — but remember, your programs need them for the PHP interpreter to recognize
Like all PHP programs, the examples in this section consist of a series of statements, each of which end with a semicolon. You can put multiple PHP statements on the same line of a program as long as they are separated with a semicolon. You can put as many blank lines between statements as you want. The PHP interpreter ignores them. The semicolon tells the interpreter that one statement is over and another is about to begin. No whitespace at all or lots and lots of whitespace between statements doesn't affect the program's execution. (Whitespace is programmer-speak for blank-looking characters such as space, tab, and newline.)
In practice, it's good style to put one statement on a line and to put blank lines between statements only when it improves the readability of your source code. The spacing in Examples Example 1-9 and Example 1-10 is bad. Instead, format your code as in Example 1-11.
In addition to ignoring whitespace between lines, the PHP interpreter also ignores
whitespace between language keywords and values. You can have zero spaces, one space,
or a hundred spaces between
Hello, World!" and again between "
Hello, World!" and the semicolon at the end of the line.
Good coding style is to put one space between
Example 1-12. Spacing
<?php print "Too many spaces" ; print"Too few spaces"; print "Just the right amount of spaces"; ?>
Language keywords (such as
number_format) are not case-sensitive. The PHP interpreter doesn't care
whether you use uppercase letters, lowercase letters, or both when you put these
keywords and function names in your programs. The statements in Example 1-13 are identical from the
As you've seen in some of the examples in this chapter, comments are a way to explain to other people how your program works. Comments in source code are an essential part of any program. When you're coding, what you are writing may seem crystal clear to you at the time. A few months later, however, when you need to go back and modify the program, your brilliant logic may not be so obvious. That's where comments come in. By explaining in plain language how the programs work, comments make programs much more understandable.
Comments are even more important when the person who needs to modify the program isn't the original author. Do yourself and anyone else who might have occasion to read your source code a favor and fill your programs with a lot of comments.
Perhaps because they're so important, PHP provides many ways to put comments in
your programs. One syntax you've seen already is to begin a line with
This tells the PHP interpreter to treat everything on that line as a comment. After
the end of the line, the code is treated normally. This style of comment is also used
// on a line after a statement to have the
remainder of the line treated as a comment. PHP also supports the Perl- and
shell-style single-line comments. These are lines that begin with
#. You can use
# to start a comment in the same places that you can use
//, but the modern style prefers
single-line comments are shown in Example
Example 1-14. Single-line comments with // or #
// This line is a comment print "Smoked Fish Soup "; print 'costs $3.25.'; # Add another dish to the menu print 'Duck with Pea Shoots '; print 'costs $9.50.'; // You can put // or # inside single-line comments // Using // or # somewhere else on a line also starts a comment print 'Shark Fin Soup'; // I hope it's good! print 'costs $25.00!'; # This is getting expensive! # Putting // or # inside a string doesn't start a comment print 'http://www.example.com'; print 'http://www.example.com/menu.php#dinner';
For a multiline comment, start the comment with
and end with
*/. Everything between the
*/ is treated
as a comment by the PHP interpreter. Multiline comments are useful for temporarily
turning off a small block of code. Example
1-15 shows some multiline comments.
Example 1-15. Multiline comments
/* We're going to add a few things to the menu: - Smoked Fish Soup - Duck with Pea Shoots - Shark Fin Soup */ print 'Smoked Fish Soup, Duck with Pea Shoots, Shark Fin Soup '; print 'Cost: 3.25 + 9.50 + 25.00'; /* This is the old menu: The following lines are inside this comment so they don't get executed. print 'Hamburger, French Fries, Cola '; print 'Cost: 0.99 + 1.25 + 1.50'; */
There is no strict rule in PHP about which comment style is the best. Multiline
comments are often the easiest to use, especially when you want to comment out a
block of code or write a few lines describing a function. However, when you want to
tack on a short explanation to the end of a line, a
//-style comment fits nicely. Use whichever comment style you feel most