add_menu_page() │ WP 1.5.0

This example shows how to add a theme settings page to the main menu of the WordPress dashboard.

<?php
add_action('admin_menu', function(){
	add_menu_page( 'Additional site settings', 'Additional settings', 'manage_options', 'site-options', 'add_my_setting', '', 4 );
} );

function add_my_setting(){
	?>
	<div class="wrap">
		<h2><?php echo get_admin_page_title() ?></h2>

		<?php
		// settings_errors() won't work on other than options pages
		if( get_current_screen()->parent_base !== 'options-general' )
			settings_errors('option_name');
		?>

		<form action="options.php" method="POST">
			<?php
				settings_fields("opt_group");     // hidden protection fields
				do_settings_sections("opt_page"); // settings section
				submit_button();
			?>
		</form>
	</div>
	<?php

}

Add a menu item that only administrators can see:

1st variant (only for plugins):

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', 'myplugin/myplugin-admin.php', '', plugins_url( 'myplugin/images/icon.png' ), 6
	);
}

In this case the menu page code should be in this file wp-content/plugins/myplugin/myplugin-admin.php:

<?php
	echo "Some page code.";
?>

2nd variant:

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', 'custompage', 'my_custom_menu_page', plugins_url( 'myplugin/images/icon.png' ), 6
	);
}

function my_custom_menu_page(){
	echo "Some page code.";
}

If you need to make sure that a menu item has not yet been added, before adding it, you can use global variable $admin_page_hooks:

global $admin_page_hooks;

if( isset($admin_page_hooks['menu_slug']) ){
	add_submenu_page( ... );
}
else {
	add_menu_page( ..., ..., ..., 'menu_slug' );
	add_submenu_page( ... );
}

This functions checks if a menu or submenu item exists by its slug.

/**
 * Finds the specified menu or submenu item item in the dashboard.
 *
 * Use after 'admin_menu' hook.
 *
 * Usage example: if( is_admin_menu_item_exists('options-general.php') ){  }
 *
 * @param  string  $handle        Menu slug. Specified in 4 parameter of add_menu_page() or add_submenu_page()
 * @param  boolean [$sub = false] Is menu or submenu ID specified?
 * @return boolean Whether a menu item exists
 */
function is_admin_menu_item_exists( $handle, $sub = false ){

	if( !is_admin() || (defined('DOING_AJAX') && DOING_AJAX) )
		return false;

	global $menu, $submenu;

	$check_menu = $sub ? $submenu : $menu;

	if( empty($check_menu) )
		return false;

	foreach( $check_menu as $k => $item ){
		if( $sub ){
			foreach( $item as $sm ){
			  if( $handle == $sm[2] )
				return true;
			}
		}
		elseif( $handle == $item[2] )
			return true;
	}

	return false;
}

Suppose we want to add an arbitrary link to the menu: /wp-admin/post.php?post=270&action=edit:

add_action( 'admin_menu', 'change_menu' );

function change_menu() {

	$post_id  = 393;
	$url_edit = get_edit_post_link( $post_id );

	if ( $url_edit ) {
		add_menu_page( '', 'My article', 'edit_posts', $url_edit, '', '', 5 );
	}
}