I was just browsing the above and with a little modification,
came up with the following which I believe to be more flexible:

<?php
function bin2hex($data) {
    $corrected = ereg_replace("[^0-9a-fA-F]","",$data);
    return pack("H".strlen($corrected),$corrected);
}
?>

This will make sure that whatever you pass, even if it is padded
at the extremeties or between pairs, should return the desired data.