write a configuration file dynamically

[eluser]sakcret[/eluser]
I need to write a configuration file dynamically, the file will create the folder config / miarchivoconfiguracion.php, define periods in the example:

Code:

$ config ['fecha_periodo_inicio'] = '2012-01-01 ';
$ config ['fecha_periodo_fin'] = '2012-10-30 ';

try $ this-> set_item ('fecha_periodo_inicio', '2012-02-01 ');
apparently does not work because not physically writes the file
anyone know how to do please hel me thanks...