Here is the function which determines whether the PNG image contains alpha or not:
<?php
function is_alpha_png($fn){
return (ord(@file_get_contents($fn, NULL, NULL, 25, 1)) == 6);
}
?>
The color type of PNG image is stored at byte offset 25. Possible values of that 25'th byte is:
* 0 - greyscale
* 2 - RGB
* 3 - RGB with palette
* 4 - greyscale + alpha
* 6 - RGB + alpha