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 !
.

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