how to create unik key in php

<?php

function NewGuid() {
    $s = strtoupper(md5(uniqid(rand(),true)));
    $guidText =
        substr($s,0,8) . '-' .
        substr($s,8,4) . '-' .
        substr($s,12,4). '-' .
        substr($s,16,4). '-' .
        substr($s,20);
    return $guidText;
}


$Guid = NewGuid();
echo $Guid;
?>