PHP: combining images as layers, casting a shadow? - php

Question

I'm creating a program to create a product image based on choices made by the customer. The product is men's tights, with or without codpiece, with or without feet, in three different styles.
To do this, I have seventeen PNG images: background, a pair of actual feet, torso and arms above the waist (all in color); leg options all in grayscale: right leg, left leg, right codpiece, left codpiece, etc., colorized as they're laid in.
Here is the specific issue I'm trying to work out: every image is transparent (except the background) and they layer up just fine. However, the codpiece casts a shadow on the leg: right now that shadow is a black gradient fading to transparent. Combined with the picture below it, the shadow becomes a large stain (see below).
I have google-d like crazy and tried everything I can think of, but I can't find the right combination of functions. Is what I want to do possible, and if so, how?
tl;dr: How do I get one image to cast a shadow on another?
// image paths are stored in an array, built based on customer choices
$dest = imagecreatefrompng("./layers/{$layers[0][0]}.png");
for($i = 0; $i < count($layers); $i++){
imagealphablending($dest, false);
imagesavealpha($dest, true);
$src = imagecreatefrompng("./layers/{$layers[$i][0]}.png");
// apply color to the current image as appropriate
$color = $colors[$layers[$i][1]] ?? '';
if($color != ''){
colorize($src, $color);
}
// lay the current image onto the image we're building
imagecopymerge_alpha($dest, $src, 0, 0, 0, 0, 4000, 4000, 100);
}