flock function

i saw this code in a book and i didn t understand why flock is being used . and when we use it isn t it like this  
(flock($file1, LOCK_EX)) ???

<?php
$idfile=fopen("connex.txt","a");
$date=time();;
flock($idfile,2);
fwrite($idfile,$date);
flock($idfile,3);
fclose($idfile);
//Lecture des données
$idfile=fopen("connex.txt","r");
flock($idfile,1);