Will user update or overwriting file before other session done in PHP?
See here for the original answer.
PHP is synchronous unless you specify otherwise, so any write operations should be queued and prevent the vast majority of write conflicts. To be extra safe, I am copying an answer [link] that explains how to do so below:
File access is simultaneous but you can use flock to block other handles from accessing the file while you perform your read/write operations. It sounds like the best solution to your problem would be to use PDO and SQLite (if available). This way you have the database handle all locking for you but do not need a dedicated database server. SQLite is entirely file based.
If SQLite is not available, you'll want to make use of flock's LOCK_EX operation, this will only allow one write stream to access the file at a time, e.g.
// create the file handle
$hnd = fopen($destpath, 'ab+');
if ($isReadOperation) {
// shared lock - simultaneous reads can happen at one time
flock($hnd, LOCK_SH);
// perform your read operation
} else {
// exclusive lock - only one write can occur at a time after all shared locks are released
flock($hnd, LOCK_EX);
// perform your read/write operation
}
// release the lock
flock($hnd, LOCK_UN);
// release the file handle
fclose($hnd);