PHP script for concatenation and versioning of external JavaScript files.

js-combine-and-version.php

<?php 
	/**
	 * Create an associative array of JavaScript files
	 * and their respective file modification times.
	 */
	$jsfiles = array(
		'first.js'  => filemtime('path/to/first.js'),
		'second.js' => filemtime('path/to/second.js'),
		'third.js'  => filemtime('path/to/third.js')
	);
 
 
	/**
	 * Encode $jsfiles array as JSON and compare to stored JSON file.
	 */
	$json = file_get_contents('path/to/jsfiles.json');
 
	if(json_encode($jsfiles) !== $json) {
		
		/**
		 * If $jsfiles array and JSON file don't match,
		 * that means one or more files have changed.
		 *
		 * Update the JSON file with the new values.
		 */
		file_put_contents('path/to/jsfiles.json', json_encode($jsfiles));
 
		/**
		 * Combine JS files and place them in a single concatenated file
		 */
		$js  = file_get_contents('path/to/first.js');
		$js .= file_get_contents('path/to/second.js');
		$js .= file_get_contents('path/to/third.js');
 
		file_put_contents('path/to/concatenated/file.js', $js);
	}
	
	/**
	 * Use the MD5 hash of the concatenated file as a query string.
	 *
	 * Query string versioning is not best practice,
	 * but 60% of the time it works every time.
	 */
	$concat_hash = hash_file('md5', 'path/to/concatenated/file.js');
?>
	<script src="path/to/concatenated/file.js?<?= $concat_hash ?>" async></script>

I also use a minifier (JShrink), but didn't include that code here.