How to check if an image has transparency using GD?

I know this is an old thread, but in my opinion it needs improvement since walking through a huge png by checking all pixels only to find out it is not transparent is a waste of time. So after some googleing I found Jon Fox's Blog and I improved his code with the help of the W3C PNG Specification further to be reliable, fast and have a minimum on memory imprint:

function IsTransparentPng($File){
    //32-bit pngs
    //4 checks for greyscale + alpha and RGB + alpha
    if ((ord(file_get_contents($File, false, null, 25, 1)) & 4)>0){
        return true;
    }
    //8 bit pngs
    $fd=fopen($File, 'r');
    $continue=true;
    $plte=false;
    $trns=false;
    $idat=false;
    while($continue===true){
        $continue=false;
        $line=fread($fd, 1024);
        if ($plte===false){
            $plte=(stripos($line, 'PLTE')!==false);
        }
        if ($trns===false){
            $trns=(stripos($line, 'tRNS')!==false);
        }
        if ($idat===false){
            $idat=(stripos($line, 'IDAT')!==false);
        }
        if ($idat===false and !($plte===true and $trns===true)){
            $continue=true;
        }
    }
    fclose($fd);
    return ($plte===true and $trns===true);
}

It doesn't look like you can detect transparency at a glance.

The comments on the imagecolorat manual page suggest that the resulting integer when working with a true-color image can actually be shifted four times total, with the fourth being the alpha channel (the other three being red, green and blue). Therefore, given any pixel location at $x and $y, you can detect alpha using:

$rgba = imagecolorat($im,$x,$y);
$alpha = ($rgba & 0x7F000000) >> 24;
$red = ($rgba & 0xFF0000) >> 16;
$green = ($rgba & 0x00FF00) >> 8;
$blue = ($rgba & 0x0000FF);

An $alpha of 127 is apparently completely transparent, while zero is completely opaque.

Unfortunately you might need to process every single pixel in the image just to find one that is transparent, and then this only works with true-color images. Otherwise imagecolorat returns a color index, which you must then look up using imagecolorsforindex, which actually returns an array with an alpha value.

I know this is old, but I just found this on the comments of the PHP docs. (link)

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

Only works for PNG images though.