Let's say you wanted to construct your site so that it showed the visitor's name at the top of every page. With our custom welcome message example above, we're halfway there already. Here are the problems we'll need to overcome to extend the example into what we need:
We need the name on every page of the site, not just on one.
We have no control over which page of our site users will view first.
The first problem isn't too hard to overcome. Once we have the user's name in a variable on one page, we can pass it with any request to another page by adding the name to the query string of all links[4]:
<a href=" echo(urlencode($_GET['name'])); ?>"> A link </a>
Notice that we've embedded PHP code right in the middle of an HTML tag. This is perfectly legal, and will work just fine. A short cut exists for those times when you simply want to echo a PHP value in the middle of your HTML code. The short cut looks like this:
<a href=" class="bold"><?=urlencode($_GET['name'])?>"> A link </a>
The tags <?= ... ?> perform the same function as the much longer code <?php echo( ... ); ?>. This is a handy short cut that I'll use several times through the rest of this book.
You're familiar with the echo function, but urlencode is probably new to you. This function takes any special characters in the string (for example, spaces) and converts them into the special codes they need to be in order to appear in the query string. For example, if the $name variable had a value of "Kevin Yank", then, as spaces are not allowed in the query string, the output of urlencode (and thus the string output by echo) would be "Kevin+Yank". PHP would then automatically convert it back when it created the $_GET variable in newpage.php.
Okay, so we've got the user's name being passed with every link in our site. Now all we need is to get that name in the first place. In our welcome message example, we had a special HTML page with a form in it that prompted the user for his or her name. The problem with this (identified by the second point above) is that we couldn't—nor would we wish to—force the user to enter our Website by that page every time he or she visited our site.
The solution is to have every page of our site check to see if a name has been specified, and prompt the user for a name if necessary[5]. This means that every page of our site will either display its content, or prompt the user to enter a name, depending on whether the $name variable is found to have a value. If this is beginning to sound to you like a good place for an if-else statement, you're a quick study!
We'll refer to pages that can decide whether to display one thing or another as multipurpose pages. The code of a multipurpose page looks something like this:
<l>
<head>
<title> Multipurpose Page Outline </title>
</head>
<body>
<?php if (condition) { ?>
<!-- HTML content to display if condition is true -->
<?php } else { ?>
<!-- HTML content to display if condition is false -->
<?php } ?>
</body>
<l>
This code may confuse you at first, but in fact this is just a normal if-else statement with HTML code sections that depend on the condition, instead of PHP statements. This example illustrates one of the big selling points of PHP: that you can switch in and out of "PHP mode" whenever you like. If you think of <?php as the command to switch into "PHP mode", and ?> as the command to go back into "normal HTML mode", the above example should make perfect sense.
There's an alternate form of the if-else statement that can make your code more readable in situations like this. Here's the outline for a multipurpose page using the alternate if-else form:
<l> <head> <title> Multi-Purpose Page Outline </title> </head> <body> <?php if (condition): ?> <!-- HTML content to display if condition is true --> <?php else: ?> <!-- HTML content to display if condition is false --> <?php endif; ?> </body> <l>
Okay, now that we have all the tools we need in hand, let's look at a sample page of our site (samplepage.php in the code archive):
<l> <head> <title> Sample Page </title> </head> <body> <?php if ( !isset($_GET['name']) ): ?> <!-- No name has been provided, so we prompt the user for one.--> <form action="<?=$_SERVER['PHP_SELF']?>" method="get"> Please enter your name: <input type="text" name="name" /> <input type="submit" value="GO" /> </form> <?php else: ?> <p>Your name: <?=$_GET['name']?></p> <p>This paragraph contains a <a href="'name'])?>" >link</a> that passes the name variable on to the next document.</p> <?php endif; ?> </body> <l>
There are two new tricks in the above code, but overall you should be fairly comfortable with the way it works. First of all, we're using a new function called isset in the condition. This function returns (outputs) a value of true if the variable it is given has been assigned a value (i.e. if a name has been provided in this example), and false if the variable does not exist (i.e. if a name has not yet been given). The exclamation mark (also known as the negation operator, or the not operator), which appears before the name of the function, reverses the returned value from true to false, or vice-versa. Thus, the form is displayed when the $_GET['name'] variable is not set.
The second new trick is the use of the variable $_SERVER['PHP_SELF'] to specify the action attribute of the <form> tag. Like $_GET, $_POST, and $_REQUEST, $_SERVER is an array variable that is automatically created by PHP. $_SERVER contains a whole bunch of information supplied by your Web server. In particular, $_SERVER['PHP_SELF'] will always be set to the URL of the current page. This gives us an easy way to create a form that, when submitted, will load the very same page, but this time with the $name variable specified.[6]
If we structure all the pages on our site in this way, visitors will be prompted for their name by the first page they attempt to view, whichever page this happens to be. Once they enter their name and click "GO", they'll be presented with the exact page they requested. The name they entered is then passed in the query string of every link from that point onward, ensuring that they are prompted only once.
[4]If this sounds like a lot of work to you, it is. Don't worry; we'll learn much more practical methods for sharing variables between pages in "Cookies and Sessions in PHP".
[5]Again, if you're dreading the thought of adding PHP code to prompt the user for a name to every page of your site, don't fret; we'll cover a more practical way to do this later.
[6]The $_SERVER array was introduced in PHP 4.1. In previous versions of PHP, these values were available in an array called $HTTP_SERVER_VARS. Also, when register_globals is set to On in the php.ini file (the default setting in PHP versions prior to 4.2), $_SERVER['PHP_SELF'] was available simply as $PHP_SELF.