Question

In WordPress, this is done:

add_action( 'after_setup_theme', 'theme_setup' );
function theme_setup() {
  ...
}

How does WordPress access the function theme_setup() given the string theme_setup?

Answer 1

Firstly, the function add_action is called with all the parameters. Code of this function is listed below. This function in not interesting, it is just a wrapper for add_filter function, which is more interesting.

function add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
    return add_filter($tag, $function_to_add, $priority, $accepted_args);
}

Function add_filter saves the data provided to a function to a global variable. Code is follows:

function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
    global $wp_filter, $merged_filters;

    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset( $merged_filters[ $tag ] );
    return true;
}

Now you have everything you need saved in global variable and you can start to call function names from function do_action, which is pretty long. But the most interesting part is on it’s end:

call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));

Function call_user_func_array is the magic. You can read more about it in PHP manual