Preventing overwrites from concurrent forms in PHP

We're setting up a system which allows a department to make edits to a record here. Their division of labor isn't clear, and they've had problems in the past where more than one individual loads data into a web form, makes edits, and then sends those edits to the record. Inevitably, the slower editor over-writes the faster editor's freshly edited data with the old data that had been loaded when the page loaded.

Currently, we have a white-board solution that would use changes to the last modified time of the data to reject the second request to write data and deliver an error message when that data POSTED.

The members of the department, however, would prefer a file-lock styled system--one where they were notified that another user was in the dataset prior to being allowed to access the data. Timeouts will inevitably be too short or too long depending on the day, and these particular users cannot be relied upon to "log out" somehow.

I'm just wondering if anyone else has implemented a solution to this, and, if so, how?

We're running PHP 5.6 on Apache 2.2 and building in Zend Framework 2.4. Not that any of that has any bearing on the question, but someone will inevitably ask.