php - Why does Wordpress add_menu not work when seperated from Initial plugin page? -

so i'm working on very basic plugin wordpress. copied following internet menu item plugin named menu.php

<?php  /* plugin name: test plugin description: test plugin demonstrate wordpress functionality author: simon lissack version: 0.1 */ defined( 'abspath' ) or die( 'no script kiddies please!' );  add_action('admin_menu', 'test_plugin_setup_menu');  function test_plugin_setup_menu(){     add_menu_page( 'test plugin page', 'test plugin', 'manage_options', 'test-plugin', 'test_init' ); }  function test_init(){     echo "<h1>hello world!</h1>"; }  ?> 

this works expected it's stand alone plugin. have different plugin

<?php  /* plugin name: plugin description: blah blah author: me version: 0.1 */ defined( 'abspath' ) or die( 'no script kiddies please!' );  fun fun database work on register_activation_hook ?> 

the problem i'd have in 1 plugin not 2 tried , deleted commenting made register plugin in menu.php

/* plugin name: plugin description: blah blah author: me version: 0.1 */ defined( 'abspath' ) or die( 'no script kiddies please!' );  include_once('menu.php'); fun fun database work ?> 

fatal error: call undefined function wp_get_current_user() in wp-includes\capabilities.php on line 1387 , error get. doing following gives me desired result

/* plugin name: plugin description: blah blah author: me version: 0.1 */ defined( 'abspath' ) or die( 'no script kiddies please!' );   add_action('admin_menu', 'test_plugin_setup_menu');  function test_plugin_setup_menu(){     add_menu_page( 'test plugin page', 'test plugin', 'manage_options', 'test-plugin', 'test_init' ); }  function test_init(){     echo "<h1>hello world!</h1>"; }  fun fun database work ?> 

the reason i'd know how work using include or require or @ least know why can't because i'm sure plugin grow , have in bunch of smaller easier me maintain files 1 big initial file.

thank ohh i've tried

<?php  /* plugin name: plugin description: blah blah author: me version: 0.1 */ defined( 'abspath' ) or die( 'no script kiddies please!' );    //  add_action('plugins_loaded','load_menu');//didn't work same error add_action('init','load_menu');//didn't work same error  function load_menu(){     include_once('menu.php');  } fun fun database work ?> 

thanks again help