PHP Reference
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Quick Reference

The PHP flock() function locks or releases a file.

<?php
$file = fopen('test.txt', 'w+');

// exclusive lock
if (flock($file, LOCK_EX)) {
    fwrite($file, 'Add this text to the file.');
    fflush($file);

    // release lock
    flock($file, LOCK_UN);
}
else {
  echo 'Error locking file!';
}

fclose($file);
?>

Output

// locks and releases "test.txt"

Syntax

flock(file, lock, block)

Parameters

ParameterDescription
fileSpecifies an open file to lock or release (required)
lockSpecifies what kind of lock to use:

  • 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 the lock

  • LOCK_NB - Avoid blocking other processes while locking
blockSet to 1 to block other processes while locking

PHP Notes:

  • When using PHP, single or double quotation marks are acceptable and work identically to one another; choose whichever you prefer, and stay consistent
  • Arrays count starting from zero NOT one; so item 1 is position [0], item 2 is position [1], and item 3 is position [2] … and so on
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