WordPress and Call to undefined function add_menu_page()

I recently got into WordPress plugin development and I would like to add a menu page (the links in the left hand side menu). Previous SO questions and the WordPress codex say that it's as simple as calling:

add开发者_运维知识库_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position );

However when I try this in my plugin setup file it tells me that the function is undefined:

PHP Fatal error:  Call to undefined function add_menu_page()

This seems like a very simple thing to do according to the documentation but I am totally baffled. Any help would be really appreciated :)

---

I don't know how your code looks but this is how I just tested and it worked:

add_action('admin_menu', 'my_menu');

function my_menu() {
    add_menu_page('My Page Title', 'My Menu Title', 'manage_options', 'my-page-slug', 'my_function');
}

function my_function() {
    echo 'Hello world!';
}

Take a look here http://codex.wordpress.org/Administration_Menus

---

You are getting this error message because either you have used the function add_menu_page outside any hook or hooked it too early.

The function add_menu_page gets capability as a third argument to determine whether or not the user has the required capability to access the menu so the function is only available when the user capability is populated therefore you should use the function in the admin_menu hook as following.

add_action( 'admin_menu', 'register_my_custom_menu_page' );

function register_my_custom_menu_page(){
    add_menu_page(  __( 'Custom Menu Title' ), 'custom menu', 'manage_options', 'custom-page-slug', 'my_custom_menu_page' );
}

function my_custom_menu_page() {
    echo __( 'This is custom menu page.' );
}

See the following WordPress codex page for information about it.

http://codex.wordpress.org/Function_Reference/add_menu_page