Remove key PHP? HOW?

I have a serial key php script, and i edited so it would delete the key but what am i doing wrong?

<?php
    /* Grab the URL variable */    $key = $_GET["key"];
    /* Connect to database and grab the keys */    mysql_connect("localhost","keys","sql123")    or die("Couldnt connect to database server");    mysql_selectdb("keys")    or die("Couldnt select database");    $query = "SELECT `key` FROM `keys`";    $result = mysql_query($query);    $num=mysql_numrows($result);
    /* Set up a loop to check if requested key is found in database */    $i=0;    while ($i < $num) {
        $validationkey = mysql_result($result,$i,"key");
            if ($validationkey == $key) {                $validation = "Success";            }
        $i++;    }
    /* If a match was found in database, echo "VALID". Else echo "ERROR" */    if($validation != "") {              $query = 'DELETE FROM `keys` WHERE key=' . $key;        mysql_query($query);        echo 'Key $key activated and deleted to prevent scamming <br><input value="valid">'    } else {        header('Location: http://freemoneypot.com/showthread.php?tid=95');    }

?>

PHP:

Everything is fine exept this:

/* If a match was found in database, echo "VALID". Else echo "ERROR" */    if($validation != "") {              $query = 'DELETE FROM `keys` WHERE key=' . $key;        mysql_query($query);        echo 'Key $key activated and deleted to prevent scamming <br><input value="valid">'    } else {        header('Location: http://freemoneypot.com/showthread.php?tid=95');    }

PHP:

Pastebin: http://pastebin.com/CdgD1yEQ

 

If the $key isn't found, $validation is undefined, so reading it [if($validation...) reads it] throws an error.