Cost of file modification time checks

For a file containing few bytes under Linux, I need only to process when it was changed since the last time it was processed. I check whether the file was changed by calling PHP clearstatcache(); filemtime(); periodically. Since the entire file will always be tiny, would it be a performance improvement to remove the call to filemtime and check for a file change by comparing the contents with the past contents? Or what is the best method for that, in terms of performance.