Do you need to find out in PHP, when a file was modified? Then you should take a look at the PHP-function filemtime. This function expects a filename as parameter and returns the modification-time of this file.
Here is an example:
< ?php echo filemtime('anyfile.html'); ?>
This PHP-script will output the timestamp of the last modification of the file anyfile.html.
You can also format this by using the date-function: