在图片上添加透明度渐变的效果

今天因为临时需要，要动态的在图片上添加透明度渐变的效果，在网上找了半天没有相应功能的算法....

////$strimgsrc = file_get_contents("http://127.0.0.1/5307754.jpg");
////$imgsrc = imagecreatefromstring($strimgsrc);
$imgsrc = imagecreatefromjpeg("5307754.jpg");
$imgsrcw = imagesx($imgsrc);
$imgsrch = imagesy($imgsrc);

$width = 30;
$x1 = 2;
$x2 = $imgsrcw - $x1 - 20;
$y1 = ($imgsrch - $width) - 2;
$y2 = $y1 + $width;

$steps = $x2 - $x1;
for($i = 0; $i {
        $alphax = round($i/($steps/127))+60;
        if($alphax >= 128)
                $alphax = 127;
        $alpha = imagecolorallocatealpha($imgsrc, 255, 255, 255, $alphax);
        imagefilledrectangle($imgsrc, ($i+$x1), $y1, ($i+$x1+1), $y2, $alpha);
}

header('content-type: image/jpeg');

imagejpeg($imgsrc);
imagedestroy($imgsrc);
?>