Filters are applied to tags by using the pipe (“|”) character. Their effect is to call a PHP function, either one which is in the standard PHP library or one which has been declared within SPIP.
[(#TAG|filter)]
[(#TAG|filter{argument2, argument3, ...})]
Whenever a filter “x” is requested, SPIP looks for a function called “filtre_x”. If it does not find one, it looks for “filtre_x_dist”, and then “x”. It then runs the function that it has found, passing any arguments. It is important to understand that the first argument sent to the filter (and therefore to the PHP function) is the result of the component to the immediate left of that filter. Thus the example above shows the filter’s parameter list as argument2, argument3, etc.