bool flock(int fp, int operation);
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 1.
To acquire an exclusive lock (writer), set operation to 2.
To release a lock (shared or exclusive), set operation to 3.
If you don't want flock() to block while locking, add 4 to operation.
flock() allows you to perform a simple reader/writer model which can be used on virtually every platform (including most Unices and even Windows).
flock() returns true on success and false on error (e.g. when a lock could not be acquired).