PHP flock() Function

The PHP flock() function is used to lock or release a file. The lock is released also by fclose(), or when stream is garbage collected.

Syntax

flock(stream, operation, would_block)

Parameters

`stream`	`Required.` Specify the file pointer to lock or release. It is typically creating by using fopen() function.
`operation`	`Required.` Specify what kind of lock to use. Possible values are: LOCK_SH - A shared lock (reader). Allow other processes to access the file. LOCK_EX - An exclusive lock (writer). Prevent other processes from accessing the file. LOCK_UN - 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.
`would_block`	`Optional.` Set to 1 to block other processes while locking.

Return Value

Returns true on success or false on failure.

Example: flock() example

Lets assume that we have a file called test.txt. In the example below, the file is opened using 'r+' mode. Then, an exclusive lock is acquired on the file pointer. After writing some content to it, the lock is released.

<?php
//open the file in r+ mode
$fp = fopen("test.txt", "r+");

//acquiring an exclusive lock
if (flock($fp, LOCK_EX)) {
  //truncating the file
  ftruncate($fp, 0);
  
  //writing some content to it
  fwrite($fp, "flock() function example\n");

  //flushing output before releasing the lock
  fflush($fp); 

  //releasing the lock
  flock($fp, LOCK_UN);    
} else {
  echo "Error locking file!";
}

//close the file
fclose($fp);

//displaying the file content
echo file_get_contents("test.txt")."\n";
?>

The output of the above code will be:

flock() function example

Example: flock() using the LOCK_NB option

The example below shows how to use LOCK_NB option on an LOCK_EX operation while using flock() function.

<?php
//open the file in r+ mode
$fp = fopen("test.txt", "r+");

//activating the LOCK_NB option on an LOCK_EX operation
if (flock($fp, LOCK_EX | LOCK_NB)) {
  //truncating the file
  ftruncate($fp, 0);
  
  //writing some content to it
  fwrite($fp, "flock() function with LOCK_NB option\n");

  //flushing output before releasing the lock
  fflush($fp); 

  //releasing the lock
  flock($fp, LOCK_UN);    
} else {
  echo "Error locking file!";
}

//close the file
fclose($fp);

//displaying the file content
echo file_get_contents("test.txt")."\n";
?>

The output of the above code will be:

flock() function with LOCK_NB option

❮ PHP Filesystem Reference