PHP coding help

I am struggling to figure out how to make the footer in my HTML PHP code show the last time the page was modified at the file level. I use this code for all my HTML files...

SO the footer.html code is ....

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Last modified:

MY PROBLEM is how can I have it that the file name chances with the file that I am currently working on? ( I don't want it just to be able to do get the index.php but the file, but the current file. I hope this makes sense. A side note for you helpers to know is that Chegg DOES NOT allow us to write comments anymore on Post Question answers. So don't expect any comments.