add_filter : Passing an array instead of the callback function?

Question

I was just reading through BuddyPress code because I want to develop a custom plugin on top of it. I encountered something very peculiar that I haven't seen before and cannot seem to find any material on.

The following code is from a protected function under a class named BP_Legacy

// Filter BuddyPress template hierarchy and look for page templates.

add_filter( 'bp_get_buddypress_template', array( $this, 
'theme_compat_page_templates' ), 10, 1 );

As far as I know, the syntax for add_filter is something like

add_filter ('hook_name', 'callback_function', $priority, $number_of_arguments)

How is it that an array has been passed instead of the callback function? How exactly would this line of code run?

Answer 1

You can pass as the callback argument anything which is callable by PHP definition, something that might actually change between PHP versions.

In this specific case the array($o,$m) type of notation indicates that the filter will call $o->$m