The code below overwrites existing content in a file:

$handle = fopen($filename, 'w') or die('Cannot open file:  '.$filename);

The code below adds new line to the existing content:

$handle = fopen($filename, 'a') or die('Cannot open file:  '.$filename);

So, the difference is in fopen($filename, 'w') and fopen($filename, 'a')