Making darkness or brightness transparent

// $url: the URL of the image you want to be processed. It can also be a local filename.
// $type: 0 for brightness, anything else for darkness.
// $mode: 0 for theoretical mode, anything else for Photoshop mode.
 
function transparent($url, $type=1, $mode=1){
// Create the old image
$old_image = imagecreatefromstring(file_get_contents($url));
 
// Create the new image of the same size as the old one
list($x_size, $y_size) = getimagesize($url);
$new_image = imagecreatetruecolor($x_size, $y_size);
 
// Turn off alpha blending and set alpha flag for our new image
imagealphablending($new_image, false);
imagesavealpha($new_image, true);
 
// Do the following for each pixel
for($y=0; $y<$y_size; $y++){
	for($x=0; $x<$x_size; $x++){
		// get it's color
		$rgb = imagecolorat($old_image, $x, $y);
		// and get the R, G and B components of the color
		$old_color = imagecolorsforindex($old_image, $rgb);
		$r = $old_color['red'];
		$g = $old_color['green'];
		$b = $old_color['blue'];
 
		// Define brightness:
		if ($mode) $brightness = max($r, $g, $b); // photoshop mode OR
		else $brightness = (max($r, $g, $b) + min($r, $g, $b)) / 2; // theoretical mode
 
		// Define darkness:
		$darkness = (255 - $brightness);
 
		// Generate transparence:
		if($type) $alpha = $darkness / 2; // for darkness OR
		else $alpha = $brightness / 2; // for brightness
 
		// Define the new color, same as the old one, but with alpha transparency
		$new_color = imagecolorallocatealpha($new_image, $r, $g, $b, $alpha);
		// Set that color to the pixel in the new image
		imagesetpixel($new_image, $x, $y, $new_color);
	}
}
 
// Output image to browser
header('Content-Type: image/png');
imagepng($new_image);
 
// Destroy all images in the end
imagedestroy($old_image);
imagedestroy($new_image);
}