WordPress and Call to undefined function add_menu_page()

Question

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 sa

Answer 1

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