Gen All 2 Char Combos

<?php
    //Bash one-line generate = curl "http://pastebin.com/raw.php?i=AUuaujGe" | php
        $comb = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_";
        for($x = 0; $x < strlen($comb); $x++)
        {
                for($y = 0; $y < strlen($comb); $y++)
                {
                        echo $comb[$x] .  $comb[$y] . "\n";
                }
        }
?>