Date: 04/16/06 (Java Web)    Keywords: no keywords

This is for WordPress Plugin and Theme developers. Don't read further if you are not one of them. To get an extra argument in your filter (say for a 'bloginfo' filter) you have to specify not 2 but actually 4 as number of arguments in add_filter (read below for details). The returned data are: first argument, [...]

Source: http://blog.taragana.com/index.php/archive/how-to-accept-multiple-parameters-in-wordpress-filters/