file locking in php

Question

I had a newcomer (the next door teenager) write some php code to track some usage on my web site. I'm not familiar with php so I'm asking a bit about concurrent file access.

My native app (on Windows), occasionally logs some data to my site by hitting the URL that contains my php script. The native app does not examine the returned data.

        $fh = fopen($updateFile, 'a') or die("can't open file");
        fwrite($fh, $ip);
        fwrite($fh, ', ');
        fwrite($fh, $date);
        fwrite($fh, ', ');
        fwrite($fh, implode(', ', $_GET));
        fwrite($fh, "\r\n");
        fclose($fh);

This is a low traffic site, and the data is not critical. But what happens if two users collide and two instances of the script each try to add a line to the file? Is there any implicit file locking in php?

Is the code above at least safe from locking up and never returning control to my user? Can the file get corrupted? If I have the script above delete the file every month, what happens if another instance of the script is in the middle of writing to the file?

Answer 1

You should put a lock on the file:

<?php
$fp = fopen($updateFile, 'w+');
if(flock($fp, LOCK_EX)) {
fwrite($fp, 'a');
flock($fp, LOCK_UN);
} else {
echo 'can\'t lock';
}

fclose($fp);