I know this topic is super old, but, in case if someone's looking for an answer, as me, I'm posting my solution.
This solution works IF you don't mind having some extra data at the beginning of your file.
Basically, the idea is to, if file is not existing, to create it and append current date at the first line.
Next, you can read the first line with fgets(fopen($file, 'r')), turn it into a DateTime object or anything (you can obviously use it raw, unless you saved it in a weird format) and voila - you have your creation date! For example my script to refresh my log file every 30 days looks like this:
if (file_exists($logfile)) {
$now = new DateTime();
$date_created = fgets(fopen($logfile, 'r'));
if ($date_created == '') {
file_put_contents($logfile, date('Y-m-d H:i:s').PHP_EOL, FILE_APPEND | LOCK_EX);
}
$date_created = new DateTime($date_created);
$expiry = $date_created->modify('+ 30 days');
if ($now >= $expiry) {
unlink($logfile);
}
}