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_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

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