PHP Function: Get a file's last modification timestamp - PHP Function - https://ryanfitton.co.uk/blog/php-function-get-a-files-last-modified-timestamp/

function.php

<?php
  //Get the file's last modified timestamp
  function lastModTimestamp($filename, $filepath = NULL, $displayV = false) {
  	//Combine filepath and filename
  	$file = $filepath . $filename;
  
  	//If this file exists
  	if (file_exists($file)) {
  		//Get the filename's last modified date
  		$lastModDate = date("Y-m-d", filemtime($file));
  
  		//Convert the last modified date into a new Date Time object
  		$lastModTimestamp = new DateTime($lastModDate);
  
  		
  		//If $displayV is not set (NULL), only return Timestamp
  		if ($displayV == false) {
  
  			//Format Date Time object into a Timestamp and return
  			return $lastModTimestamp->getTimestamp();
  
  
  		//If $displayV is set as "true", return '?v=' and Timestamp
  		} elseif ($displayV == true) {
  
  			//Add '?v=', format Date Time object into a Timestamp and return
  			return "?v=" . $lastModTimestamp->getTimestamp();
  
  
  		//If anything else, something is broken when checking $displayV, do not display anything
  		} else {
  			return false;
  		}
  	
  
  	//If file is not found, do not display anything
  	} else {
  		return false;
  	}
  }
?>