Adds a callback function to a filter hook.

WordPress offers filter hooks to allow plugins to modify
various types of internal data at runtime.

A plugin can modify data by binding a callback to a filter hook. When the filter
is later applied, each bound callback is run in order of priority, and given
the opportunity to modify a value by returning a new value.

The following example shows how a callback function is bound to a filter hook.

Note that `$example` is passed to the callback, (maybe) modified, then returned:

function example_callback( $example ) {
// Maybe modify $example in some way.
return $example;
}
add_filter( ‘example_filter’, ‘example_callback’ );

Bound callbacks can accept from none to the total number of arguments passed as parameters
in the corresponding apply_filters() call.

In other words, if an apply_filters() call passes four total arguments, callbacks bound to
it can accept none (the same as 1) of the arguments or up to four. The important part is that
the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
opted to accept. If no arguments were accepted by the callback that is considered to be the
same as accepting 1 argument. For example:

// Filter call.
$value = apply_filters( ‘hook’, $value, $arg2, $arg3 );

// Accepting zero/one arguments.
function example_callback() {
…
return ‘some value’;
}
add_filter( ‘hook’, ‘example_callback’ ); // Where $priority is default 10, $accepted_args is default 1.

// Accepting two arguments (three possible).
function example_callback( $value, $arg2 ) {
…
return $maybe_modified_value;
}
add_filter( ‘hook’, ‘example_callback’, 10, 2 ); // Where $priority is 10, $accepted_args is 2.

*Note:* The function will return true whether or not the callback is valid.
It is up to you to take care. This is done for optimization purposes, so
everything is as quick as possible.

function add_filter( $hook_name, $callback, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter;

	if ( ! isset( $wp_filter[ $hook_name ] ) ) {
		$wp_filter[ $hook_name ] = new WP_Hook();
	}

	$wp_filter[ $hook_name ]->add_filter( $hook_name, $callback, $priority, $accepted_args );

	return true;
}