Crop Image as shape with alpha transparency using PHP GD

gistfile1.php

<?php
    
    /**
     * PHP GD use one image to mask another image including transparency
     * @param (GD)resource $picture orignal reference image
     * @param (GD)resource $mask shape black/white image
     * @return (GD)resource output shape image resource
     *
     * http://stackoverflow.com/questions/7203160/
     * 
     */
    
    private function _cropShape(&$picture, $mask) {
        // Get sizes and set up new picture
        $xSize = imagesx($picture);
        $ySize = imagesy($picture);
        $newPicture = imagecreatetruecolor($xSize, $ySize);
        imagesavealpha($newPicture, true);
        imagefill($newPicture, 0, 0, imagecolorallocatealpha($newPicture, 255, 255, 255, 127));

        // Resize mask if necessary
        if ($xSize != imagesx($mask) || $ySize != imagesy($mask)) {
            $tempPic = imagecreatetruecolor($xSize, $ySize);
            imagecopyresampled($tempPic, $mask, 0, 0, 0, 0, $xSize, $ySize, imagesx($mask), imagesy($mask));
            imagedestroy($mask);
            $mask = $tempPic;
        }

        // Perform pixel-based alpha map application
        for ($x = 0; $x < $xSize; $x++) {
            for ($y = 0; $y < $ySize; $y++) {
                $alpha = imagecolorsforindex($mask, imagecolorat($mask, $x, $y));
                $alpha = 127 - floor($alpha['red'] / 2);
                $color = imagecolorsforindex($picture, imagecolorat($picture, $x, $y));
                imagesetpixel($newPicture, $x, $y, imagecolorallocatealpha($newPicture, $color['red'], $color['green'], $color['blue'], $alpha));
            }
        }


        $picture = $newPicture;
    }
    
?>