WordPress function to show the "next-page" button in the tinyMCE

function.php

// Add "Next page" button to TinyMCE
function add_next_page_button( $mce_buttons ) {
	$pos = array_search( 'wp_more', $mce_buttons, true );
	if ( $pos !== false ) {
		$tmp_buttons = array_slice( $mce_buttons, 0, $pos+1 );
		$tmp_buttons[] = 'wp_page';
		$mce_buttons = array_merge( $tmp_buttons, array_slice( $mce_buttons, $pos+1 ) );
	}
	return $mce_buttons;
}
add_filter( 'mce_buttons', 'add_next_page_button' );