Using imagecolorallocatealpha() to Allocate Colors with Alpha in PHP GD

The PHP function imagecolorallocatealpha() assigns a color with an alpha (transparency) value to an image resource, similar to imagecolorallocate() but with an extra $alpha parameter ranging from 0 (fully opaque) to 127 (fully transparent).

It returns the allocated color index on success, or FALSE (or a falsy non‑boolean) on failure.

The example creates a 300×300 true‑color image, allocates background and border colors, defines three overlapping circles with different colors and alpha levels, outputs the result as a PNG image, and finally destroys the image resource.

<?php
$size = 300;
$image = imagecreatetruecolor($size, $size);
// something to get a white background with black border
$back = imagecolorallocate($image, 255, 255, 255);
$border = imagecolorallocate($image, 0, 0, 0);
imagefilledrectangle($image, 0, 0, $size-1, $size-1, $back);
imagerectangle($image, 0, 0, $size-1, $size-1, $border);
$yellow_x = 100;
$yellow_y = 75;
$red_x = 120;
$red_y = 165;
$blue_x = 187;
$blue_y = 125;
$radius = 150;
// allocate colors with alpha values
$yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
$red = imagecolorallocatealpha($image, 255, 0, 0, 75);
$blue = imagecolorallocatealpha($image, 0, 0, 255, 75);
// drawing 3 overlapped circle
imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);
imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);
imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);
// don't forget to output a correct header!
header('Content-Type: image/png');
// and finally, output the result
imagepng($image);
imagedestroy($image);
?>