flock

flock() allows you to perform a simple reader/writer model which can be used on virtually every platform (including most Unix derivatives and even Windows).

On versions of PHP before 5.3.2, the lock is released also by fclose() (which is also called automatically when script finished).

PHP supports a portable way of locking complete files in an advisory way (which means all accessing programs have to use the same way of locking or it will not work). By default, this function will block until the requested lock is acquired; this may be controlled with the LOCK_NB option documented below.

A file system pointer resource that is typically created using fopen().

operation is one of the following:

• LOCK_SH to acquire a shared lock (reader).
• LOCK_EX to acquire an exclusive lock (writer).
• LOCK_UN to release a lock (shared or exclusive).

It is also possible to add LOCK_NB as a bitmask to one of the above operations if you don't want flock() to block while locking.

The optional third argument is set to 1 if the lock would block (EWOULDBLOCK errno condition).

Returns TRUE on success or FALSE on failure.

Added support for the wouldblock parameter on Windows.

The automatic unlocking when the file's resource handle is closed was removed. Unlocking now always has to be done manually.

flock() uses mandatory locking instead of advisory locking on Windows. Mandatory locking is also supported on Linux and System V based operating systems via the usual mechanism supported by the fcntl() system call: that is, if the file in question has the setgid permission bit set and the group execution bit cleared. On Linux, the file system will also need to be mounted with the mand option for this to work.

Because flock() requires a file pointer, you may have to use a special lock file to protect access to a file that you intend to truncate by opening it in write mode (with a "w" or "w+" argument to fopen()).

May only be used on file pointers returned by fopen() for local files, or file pointers pointing to userspace streams that implement the streamWrapper::stream_lock() method.

<?php

$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) {  // acquire an exclusive lock
    ftruncate($fp, 0);      // truncate file
    fwrite($fp, "Write something here\n");
    fflush($fp);            // flush output before releasing the lock
    flock($fp, LOCK_UN);    // release the lock
} else {
    echo "Couldn't get the lock!";
}

fclose($fp);

?>

<?php
$fp = fopen('/tmp/lock.txt', 'r+');

/* Activate the LOCK_NB option on an LOCK_EX operation */
if(!flock($fp, LOCK_EX | LOCK_NB)) {
    echo 'Unable to obtain lock';
    exit(-1);
}

/* ... */

fclose($fp);
?>