auto_version.php

<?php
  
/**
 *  Given a valid file location (it must be an path starting with "/"), i.e. "/css/style.css",
 *  it returns a string containing the file's mtime as query string, i.e. "/css/style.css?v=0123456789".
 *  Otherwise, it returns the file location.
 *
 *  @param $file  the file to be loaded.
 */
function auto_version($file) {
    // if it is not a valid path (example: a CDN url)
    if (strpos($file, '/') !== 0 || !file_exists($_SERVER['DOCUMENT_ROOT'] . $file)) return $file;

    // retrieving the file modification time
    // https://www.php.net/manual/en/function.filemtime.php
    $mtime = filemtime($_SERVER['DOCUMENT_ROOT'] . $file);

    return sprintf("%s?v=%d", $file, $mtime);
}

css_import.php

<?php

const CSS = array(
	'/css/myCSSFile1.css',
	'/css/myCSSFile2.css',
	'/css/myCSSFile3.css',
);

foreach (CSS as $css)	
  echo '<link rel="stylesheet" href="' . auto_version($css) . '" type="text/css">';

js_import.php

<?php

const JS = array(
	'/js/myJsFile1.js',
	'/js/myJsFile2.js',
	'/js/myJsFile3.js',
);

foreach (JS as $js)	
	echo '<script src="' . auto_version($js) . '" type="text/javascript"></script>';