Cropping Image to larger than original size - php

Question

I am trying to crop an image, using some part of the image but also allowing for 'extra' space to be added around it. However when the cropped image generates black space in the 'extra' space, when I want it to be transparent.
Using cropper JavaScript to get crop coordinates: https://fengyuanchen.github.io/cropperjs/
Then use of PHP imagecopyresampled to crop the image to size.
The cropping of the image is fine, however if I crop an image to larger than the original size it adds black space around the image, I want to change this to transparent.
Looked into searching the croppped image for black pixels and converting them to transparent, however this idea breaks when the image has a black in it
Current php code: (asuming file type is PNG)
//$cropData
//is an array of data passed through from the cropper containing the original width and height, new width and height and the cropping x and y coordinates.
//passed in image to be cropped
$current_image = "/folder/example.jpg";
//image file location of cropped image
$image_name = "/folder/cropped_example.jpg";
//create blank image of desired crop size
$width = $cropData["width"];
$height = $cropData["height"];
$background = imagecreatetruecolor($width, $height);
//crop coordinates
$crop_x = $cropData["x"];
$crop_y = $cropData["y"];
//create resouce image of current image to be cropped
$image = imagecreatefrompng($current_image);
//crop image
imagecopyresampled($background, $image, 0, 0, $crop_x, $crop_y, $width, $height, $width, $height)){
imagepng($background, $image_name);
//File Uploaded... return to page

Answer 1

First you need to enable alpha channel by passing true to imagesavealpha
Next step is to disable alphablending by passing false to imagealphablending Otherwise the alpha channel will be used to recalculate colors and its value will be lost.
Allocate a transparent color passing 127 as alpha value to imagecolorallocatealpha
Fill the background of the source image by this color (e.g. calling imagefilledrectangle)
When passing source width and height parameters to imagecopyresampled do not exceed the real size of the image, otherwise out-of-bounds area will be assumed as opaque black.
Example:
$background = imagecreatetruecolor($width, $height);
//crop coordinates
$crop_x = 10;
$crop_y = 10;
imagesavealpha($background, true); // alpha chanel will be preserved
imagealphablending($background, false); // disable blending operations
$transparent_color = imagecolorallocatealpha($background, 0, 0, 0, 127); // allocate transparent
imagefilledrectangle($background, 0, 0, $width, $height, $transparent_color); // fill background
//create resouce image of current image to be cropped
$image = imagecreatefrompng($current_image);
// Limit source sizes;
$minw = min($width, imagesx($image));
$minh = min($height, imagesy($image));
//crop image
imagecopyresampled($background, $image, 0, 0, $crop_x, $crop_y, $minw, $minh, $minw, $minh);
imagepng($background, $image_name);
// done!