Need to take a bunch of pngs and create a single png that can be css animated?

make_png_sequence.php

<?php
/**
 * User: Matthew Augier
 * Data Product Services Ltd.
 * PHP command line code to convert a sequence of PNG's into a single png file
 * which can be used with css to play it as an animation
 * Date: 05/07/2016
 * Time: 21:46
 */

function human_filesize($bytes, $decimals = 2) {
	$sz = 'BKMGTP';
	$factor = floor((strlen($bytes) - 1) / 3);
	return sprintf("%.{$decimals}f", $bytes / pow(1024, $factor)) . @$sz[$factor];
}

// help!
if (count($argv) == 1) {
	die("php make_png_sequence.php [folder] [compression 0-9, 0=none]");
}

// get the path to look for from the command line
$folder = isset($argv[1]) ? $argv[1] : 'mooo';
if ( ! file_exists($folder)) die("Unable to find the folder " . $folder);
$compression = isset($argv[2]) ? $argv[2] : 2;

// get a list of png files
$files = [];
foreach (glob($folder . "/*.png") as $filename) {
	$files[] = $filename;
}

// sort just in casef
sort($files);

// load the first file to get info
$frames = count($files);
$info = getimagesize($files[0]);
list($width, $height) = $info;
print "Image details - Width:{$width}, Height:{$height}, Frames:{$frames} \r\n";
$out_width = $width * $frames;
print "Output image - Width:{$out_width}, Height:{$height}\r\n";
$css_frames = $frames - 1 ;
$bg_pos = $out_width - $width;
print "CSS helpers -  #id {animation: play 5s steps({$css_frames}) infinite;} @keyframes play {100% { background-position: -{$bg_pos}px; }}\r\n";

// increase the memory limit due to image size
ini_set('memory_limit','512M');
$out_image = imagecreatetruecolor($out_width, $height);
$offset = 0;
foreach($files as $file){
	$frame = imagecreatefrompng($file);
	imagecopy($out_image, $frame, $offset, 0, 0, 0, $width, $height);
	$offset += $width;
}

// save the image out
imagepng($out_image, $folder . '.png', $compression);

print "Completed writing {$folder}.png - filesize " . human_filesize(filesize($folder . '.png'));

Folder is the folder with a sequence of png images, output will be the folder name .png - Note this is designed to run as a command line, would need work to use for anything production wise