7 - PHP:Introduction - Getting more information.
Let us do something more useful now. We are going to check what sort of browser the visitor is using. For that, we check the user agent string the browser sends as part of the HTTP request. This information is stored in a variable. Variables always start with a dollar-sign in PHP. The variable we are interested in right now is $_SERVER['HTTP_USER_AGENT'].
Note: $_SERVER is a special reserved PHP variable that contains all web server information. It is known as an autoglobal (or superglobal).
To display this variable, you can simply do:
<?php echo $_SERVER['HTTP_USER_AGENT']; ?>
A sample output of this script may be:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
There are many types of variables available in PHP. In the above example we
printed an Array element. Note the use of square brackets[ ] to indicate the
array, as in JavaScript. Arrays can be very useful.
$_SERVER is just one variable that PHP automatically makes available to you.
You can put multiple PHP statements inside a PHP tag and create little blocks of code that do more than just a single echo. For example, if you want to check for Internet Explorer you can do this:
<?php
if (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== FALSE) {
echo 'You are using Internet Explorer.<br />';
}
?>
A sample output of this script may be:
You are using Internet Explorer.<br />
Here we introduce a couple of new concepts. We have an if statement.
It works just as it does in JavaScript.
The second concept we introduced was the strpos() function call. strpos() is a function built into PHP which searches a string for another string. In this case we are looking for 'MSIE' (so-called needle) inside $_SERVER['HTTP_USER_AGENT'] (so-called haystack). If the needle is found inside the haystack, the function returns the position of the needle relative to the start of the haystack. Otherwise, it returns FALSE. If it does not return FALSE, the if expression evaluates to TRUE and the code within its {braces} is executed. Otherwise, the code is not run. The strpos() function show us that PHP has string handling capablilities similar to JavaScript.
We can take this a step further and show how you can jump in and out of PHP mode even in the middle of a PHP block:
<?php
if (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== FALSE) {
?>
<h3>strpos() must have returned non-false</h3>
<p>You are using Internet Explorer</p>
<?php
} else {
?>
<h3>strpos() must have returned false</h3>
<p>You are not using Internet Explorer</p>
<?php
}
?>
A sample output of this script may be:
<h3>strpos() must have returned non-false</h3>
<p>You are using Internet Explorer</p>
Instead of using a PHP echo statement to output something, we jumped out of
PHP mode and just sent straight HTML. The important and powerful point to note
here is that the logical flow of the script remains intact. Only one of the
HTML blocks will end up getting sent to the viewer depending on the result of
strpos(). In other words, it depends on whether the string MSIE was found or
not.
(from php.net)