Be aware that

<?php
$rgba     = imagecolorat($image, $x, $y);
$r = ($rgba >> 16) & 0xFF;
$g = ($rgba >> 8) & 0xFF;
$b = $rgba & 0xFF;
$a     = ($rgba & 0x7F000000) >> 24;
?> 

will only work for truecolor images. With eg GIF images, this will have strange results. For GIF images, you should always use imagecolorsforindex().