identify-id.php

<?php
/*
Plugin Name: Identify ID
*/

add_filter( 'the_content' , 'mh_identify_id' );
function mh_identify_id( $content ) {
        $mh_identify = '<h3>The ID is ' . get_the_ID();
        if( is_page( get_the_ID() ) ) {
                $mh_identify .= ' and this is a page.</h3>';
                } else {
                $mh_identify .= ' and this is a post.</h3>';
        }
        return $mh_identify . $content;
}