gistfile1.php

<?php
//error_reporting(E_ERROR);

$original = imagecreatefrompng("assets/picture.png");
$mask = imagecreatefrompng("assets/mask.png");

$masked = applyAlphaMask( $original, $mask );

function applyAlphaMask($image, $mask) {
  
	$pictureWidth = imagesx($image);
	$pictureHeight = imagesy($image);
	
	$canvas = imagecreatetruecolor($pictureWidth, $pictureHeight);

	imagesavealpha($canvas, true);
	imagefill($canvas, 0, 0, imagecolorallocatealpha($canvas, 0, 0, 0, 127));

	for($x = 0; $x < $pictureWidth; $x++) {
	    for($y = 0; $y < $pictureHeight; $y++) {

	        $alphaAt = imagecolorsforindex($mask, imagecolorat( $mask, $x, $y ));

			if(($alphaAt['red'] == 0) && ($alphaAt['green'] == 0) && ($alphaAt['blue'] == 0) && ($alphaAt['alpha'] == 0)) {
				imagesetpixel($canvas, $x, $y, imagecolorallocatealpha($canvas, 0, 0, 0, 127));
			} else {
				$colorAt = imagecolorsforindex($image, imagecolorat($image, $x, $y));
				imagesetpixel($canvas, $x, $y, imagecolorallocatealpha($canvas, $colorAt['red'], $colorAt['green'], $colorAt['blue'], $colorAt['alpha']));
			}

	    }
	}

	return $canvas;

}

header("Content-type: image/png");
imagepng($masked);