Sorry, below should be:
<?php
$is_alpha = ord (file_get_contents ($file_path, false, null, 25, 1)) & 4;
?>
much better :)
Both types 4 (greyscale transparent png), and 6 (colour transparent png) have bit 4 set, so there's no need to bitwise twice and there's no need for == as the return value will be 4 or 0, which is easily interpreted as true or false elsewhere in your code.
note: file_get_contents isn't reading the whole file in this instance, just 1 byte on its own and that's it, so you can be assured this is fast and safe.
