Script not working, no errors though.

[{yak}]

You don't need to echo $contents in your function, unless you want it to show up twice. Also, I doubt it's like the '+' and '-' signs in the query string, try something different:

<?php

function open_file($filetype,$increment,$filename)
{
    $handle = fopen($filename, 'r');
    $contents = fread($handle,filesize($filename));
    fclose($handle);
        If($increment == 'minus'){
          $contents--;
        }else{
          $contents++;
        }
    $handle = fopen($filename, $filetype);
    fwrite($handle,$contents);
    fclose($handle);
    return $contents;
}
Switch($_GET['user']){
    Case "plus":
        echo open_file('w','plus','users.txt');
        break;
    Case "minus":
        echo open_file('w','minus','users.txt');
        break;
}


?>