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                                     flock

   (PHP 3>= 3.0.7, PHP 4 )
   flock -- Portable advisory file locking

Description

   bool flock ( resource handle, int operation [, int &wouldblock])

   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 handle 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)

   Returns TRUE on success or FALSE on failure.

   Example 1. flock() example
   <?php
   $fp = fopen("/tmp/lock.txt", "w+");
   if (flock($fp, LOCK_EX)) { // do an exclusive lock
       fwrite($fp, "Write something here\n");
       flock($fp, LOCK_UN); // release the lock
   } else {
       echo "Couldn't lock the file !";
   }
   fclose($fp);
   ?>

     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()).

   Warning

   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!

   flock() is not supported on antiquated filesystems like FAT and its
   derivates and will therefore always return FALSE under this
   environments (this is especially true for Windows 98 users).
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