PHP automatically creates variables for all the data it receives in an HTTP request. This can include GET data, POST data, cookie data, and environment variables. The variables are either in PHP's global symbol table or in one of a number of superglobal arrays, depending on the value of the register_globals setting in your php.ini file.
In PHP 4.2.0 and after, the default setting for register_globals is off. With register_globals off, all the various variables that are usually available directly in the global symbol table are now available via individual superglobal arrays. There is a limited set of superglobals and they cannot be created from a user-level script. The superglobal array to use depends on the source of the variable. Here is the list:
$_GET
GET-method variables. These are the variables supplied directly in the URL. For example, with http://www.example.com/script.php?a=1&b=2, $_GET['a'] and $_GET['b'] are set to 1 and 2, respectively.
$_POST
POST-method variables. Form field data from regular POST-method forms.
$_COOKIE
Any cookies the browser sends end up in this array. The name of the cookie is the key and the cookie value becomes the array value.
$_REQUEST
This array contains all of these variables (i.e., GET, POST, and cookie). If a variable appears in multiple sources, the order in which they are imported into $_REQUEST is given by the setting of the variables_order php.ini directive. The default is 'GPC', which means GET-method variables are imported first, then POST-method variables (overriding any GET-method variables of the same name), and finally cookie variables (overriding the other two).
$_SERVER
These are variables set by your web server. Traditionally things like DOCUMENT_ROOT, REMOTE_ADDR, REMOTE_PORT, SERVER_NAME, SERVER_PORT, and many others. To get a full list, have a look at your phpinfo( ) output, or run a script like the following to have a look:
<?php foreach($_SERVER as $key=>$val) { echo '$_SERVER['.$key."] = $val<br>\n"; }?>
$_ENV
Any environment variables that were set when you started your web server are available in this array.
$_FILES
For RFC 1867-style file uploads the information for each uploaded file is available in this array. For example, for a file upload form containing:
<input name="userfile" type="file">
The $_FILES array will look something like this:
$_FILES['userfile']['name'] => photo.png$_FILES['userfile']['type'] => image/png$_FILES['userfile']['tmp_name'] => /tmp/phpo3kdGt$_FILES['userfile']['error'] => 0$_FILES['userfile']['size'] => 158918
Note that the 'error' field is new for PHP 4.2.0 and the values are: 0 (no error, file was uploaded); 1 (the uploaded file exceeds the upload_max_filesize directive in php.ini); 2 (the uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form); 3 (the actual number of bytes uploaded was less than the specified upload file size); and 4 (no file was uploaded).