Why should you lock the file every time when you write or read data? Cause two instances of your script could be writing data to the same file in the same time.

PHP File locking

Learn how to use PHP file locking.

You need to lock the file each time when you do any input/output operation with that file, for example, guest book or forum or counter script.
It seems to be all right, when you simple open file and read/write data from it like this.

$fp=fopen("text",'a+');
    fwrite($fp, "$REMOTE_ADDR\n\r");
fclose($fp)

But if you use this code in any web script (for guestbook or forum or something), there is probability for this script to crash with errors or even to crash deleting all the data previously stored in the file. Because you can't avoid the situation when two users simultaneously add and retreive data from the file, for example getting and adding data to a guestbook in one time. To avoid this, you should use file locking procedure. The existance and internal structure of this procedure depends on operation system where you use PHP.

bool flock (int fp, int operation [, int wouldblock])
where
  fp: file handle
  operation can be:
     LOCK_SH - block for reading (1)
     LOCK_EX - block for writing (2)
     LOCK_UN - unblock (3)
  wouldblock: non necessary field (set LOCK_NB, to not block while locking)

Using this function you can secure your application from crashes when it is executing input/output operation. Modified code that reads data from a file looks like this:

$filename="text.txt";
$fp=fopen($filename,'r');
  flock($fp, LOCK_SH);
    $contents = fread($fp, filesize($filename));
  flock($fp, LOCK_UN);
fclose($fp);

To write data into the file using lock write something like this:

$filename="text.txt";
$fp=fopen($filename,'r');
  flock($fp, LOCK_EX);
    fwrite($fp, "$REMOTE_ADDR\n\r");
  flock($fp, LOCK_UN);
fclose($fp);

There is also one feature in using file locking function. You should lock/unlock file in any script that deals with that file. So if you have for example one script adding data to a file and another script showing file contents, if you omit file locking in any of the two your locking operations will be useless. So it is highly recommended to use file locking anytime you execute input / output operation in your web application.
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