Get all divs with specific class and add

I am trying to collect all div's with a specific class and add a to it. This file includes an overlay, which gathers the users information from a database. Initially I tried adding the file with javascript's innerHtml, but this obviously wont work since it won't run server side. The div's will be added when the user creates an item in the backend/cms and for this reason it is unknown how many div's there will be and how many overlays need to be added. If you guys have any other suggestions for adding the content of a .php file please recommend them.

I hope someone can help me out!