Send info to the server when someone leaves a specific page

Send info to the server when someone leaves a specific page

I'm coding a translation system for multiple messages.
I've a page with a table listing all the messages that need to be
translated. When a translator click on a message to translate it, I lock
it cause no other translator can be assigned to it.
The problem arises when a translator enters and instead to write
something, leaves the page in an unconventional way like back button or
closing the browser.
If this happens I want to unlock the message to make it available again to
other translators.
How can I reach this behavior?
I know the javascript onbeforeunload event but triggers also every time a
user refresh the page and this isn't what I want.
ty in advance
EDIT: seems that implementing a js ajax call to notify every minute the
server is the way to go. Now the question is how to handle the PHP server
side?