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).
flock() operates on fp which must be an open file pointer. operation is one of the following values:
To acquire a shared lock (reader), set operation to LOCK_SH (set to 1 prior to PHP 4.0.1).
To acquire an exclusive lock (writer), set operation to LOCK_EX (set to 2 prior to PHP 4.0.1).
To release a lock (shared or exclusive), set operation to LOCK_UN (set to 3 prior to PHP 4.0.1).
If you don't want flock() to block while locking, add LOCK_NB (4 prior to PHP 4.0.1) to operation.
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 optional third argument is set to TRUE if the lock would block (EWOULDBLOCK errno condition)
flock() returns TRUE on success and FALSE on error (e.g. when a lock could not be acquired).
Note: 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()).
flock() will not work on NFS and many other networked file systems. Check your operating system documentation for more details.
On some operating systems flock() is implemented at the process level. When using a multithreaded server API like ISAPI 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!