The flock Function

The flock function sets or removes a file lock. Its first parameter is a file pointer obtained via fopen, the second parameter is the lock type (LOCK_SH, LOCK_EX, or LOCK_UN), and the third optional parameter is a flag for non-blocking operation.

Syntax

phpflock(resource $handle, int $operation, int &$would_block = null): bool

Example

Locking a file for exclusive access (writing):

php<?php
  $file = fopen("data.txt", "c+");
  if (flock($file, LOCK_EX)) {
    ftruncate($file, 0);
    fwrite($file, "New data");
    flock($file, LOCK_UN);
  }
  fclose($file);
?>

Example

Shared lock for reading:

php<?php
  $file = fopen("data.txt", "r");
  if (flock($file, LOCK_SH)) {
    $content = fread($file, filesize("data.txt"));
    flock($file, LOCK_UN);
  }
  fclose($file);
  echo $content;
?>

Example

Non-blocking lock acquisition attempt:

php<?php
  $file = fopen("data.txt", "c+");
  if (flock($file, LOCK_EX | LOCK_NB, $would_block)) {
    if ($would_block) {
      echo "File is locked by another process";
    } else {
      fwrite($file, "Data");
      flock($file, LOCK_UN);
    }
  }
  fclose($file);
?>

See Also