Lock a file in php

lock.php

<?php
  $file = fopen("test.txt","w+");
  // exclusive lock
  if (flock($file,LOCK_EX))
  {
    fwrite($file,"Write something");
    // release lock
    flock($file,LOCK_UN);
  }
  else
  {
    echo "Error locking file!";
  }
  fclose($file);
?>