# Variables and Constants for customisation

Some behaviours of the pages on your site can be modified using PHP variables or constants. These variables or constants are normally defined by SPIP, but to provide more delicate customisations of the site, the webmaster has the capability to modify them.
Most of these customisations are defined in the mes_options.php file that you must carefully be located in the config/ directory of the site.
This file must be a PHP file starting with <?php and ending with ?> (remember not to leave spaces or blanks lines before <?php or after ?>).