WordPress filter hooks and how to define them in example

In WordPress, a filter is a function. Developers can create custom filters by using the Filter API. 

This process is called “hooking” and the place where we apply the filter is called a hook.

Here you can find some more examples of using filters and actions in general and some explanation about the differences between these two. Here is the special filter example that is very easy to understand. The most important part is to create a hook. Actually we create a hook by using the apply_filters special WordPress function. Somewhere if our theme files we may have this code:

echo apply_filters( 'my_custom_filter_hook', 'Copyright 2013 by programming-review' );

Please note the most important code sequence in here echo apply_filters actually this is what defines hook entry point. Here is our replace_copyright() function being set to my_custom_filter_hook hook

function replace_copyright( $copyright ) {
// do something to $copyright
return $copyright;
}
add_filter( 'my_custom_filter_hook', 'replace_copyright' );