include ('classes/config.php');
include_once ('classes/config.php');
require ('classes/config.php');
require_once ('classes/config.php');

Included Files Corrupting Things

Whitespace before or after the php start and end markers

White space or any characters before or after the <?php and ?> will be output in the same way as echo(as html) in an included file. This can be a problem, for instance having a line break after the ?> and the file being included before say the start of say an echo'd xml output will corrupt the xml output as a valid xml document requires the first line XML definition at the top of the file before any line break.