Using $_GET for POST

In handling an HTTP POST request I came across some PHP code, which I need to modify for my own purposes, which has code like this: if ( ! (isset($_GET['xxxxx']) && $_GET['xxxxx'] == 20) ) { // Do something by returning an error } Can this ever be correct when the form looks like: <form ENCTYPE="multipart/form-data" ACTION="" METHOD="POST"> <input NAME="SomeFile" TYPE="file"> <input VALUE="submit" TYPE="submit"></form> ? Is the $_GET possibly being used to check for an 'xxxxx' parameter being passed in the query part of the URL ? I am fairly new to PHP so I am trying to understand how $_GET differs from $_POST. Thanks !