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).
The lock is released also by fclose, or when stream is garbage collected.
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 flock should not block during the locking attempt.
The optional third argument is set to 1 if the lock would block (EWOULDBLOCK errno condition).
Returns true on success or false on failure.
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.
Assigning another value to stream argument in subsequent code will release the lock.
On some operating systems flock is implemented at the process level. When using a multithreaded server API you may not be able to rely on flock to protect files against other PHP scripts running in parallel threads of the same server instance!
flock is not supported on antiquated filesystems like FAT and its derivates and will therefore always return false under these environments.
On Windows, if the locking process opens the file a second time, it cannot access the file through this second handle until it unlocks the file.