alphatest.php

<?php
$im = imagecreatefrompng('alphatest.png');
$size = min(imagesx($im), imagesy($im));
$im2 = imagecrop($im, ['x' => 0, 'y' => 0, 'width' => $size, 'height' => $size]);
if ($im2 !== FALSE) {
	imagepng($im2, 'example-cropped.png');
	imagedestroy($im2);
}
imagedestroy($im);