Writing into the file in PHP

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')