PHP reference

flock

Locks a file so that two processes do not work with it at once.

Syntax

flock(resource $stream, int $operation, int &$would_block = null): bool

Parameters

Parameter	Type	Description
$stream	resource	An open file.
$operation	int	`LOCK_SH` for reading, `LOCK_EX` for writing, `LOCK_UN` to release.
$would_block	int	Optional. Set when the lock would have to wait.

Return value

bool`true` if the lock was acquired.

Examples

A counter protected by a lock:

<?php
 
$file = sys_get_temp_dir() . '/ref-counter.txt';
file_put_contents($file, '41');
 
$handle = fopen($file, 'c+');
flock($handle, LOCK_EX);
 
$count = (int) fread($handle, 20) + 1;
ftruncate($handle, 0);
rewind($handle);
fwrite($handle, (string) $count);
 
flock($handle, LOCK_UN);
fclose($handle);
 
echo file_get_contents($file);

Output:

Notes

Without a lock two simultaneous requests read the same value and one of them is lost. For a simple write there is file_put_contents($file, $data, LOCK_EX).

In the tutorial

Reading and writing files — Working with files in PHP — writing, reading, appending, reading line by line, checking existence and deleting.

Thanks for reading!