Combine two images in Php - one top of another

Question

I'm trying to create a photo collage in php by combining two images, a background and an image.

Certain part of my background is transparent, I want to put the image into that part of the background and merge them. The transparent part could be a square, circle, oval rectangle or any other shape. I tried the following code, but it doesn't work

$image_1 = imagecreatefrompng('images/background.png');
$image_2 = imagecreatefrompng('images/image.png');
imagealphablending($image_1, true);
imagesavealpha($image_1, true);
imagecopy($image_1, $image_2, 0, 0, 0, 0, 100, 100);
imagepng($image_1, 'images/3.png');

The code above combines the two images, but with one of them being very small, and the other take up the whole screen which is not what I wanted. I need to have the the image fit into the transparent part of the background of the newly combined image.