Override page templates in WordPress

functions.php

<?php
/*
 * Override the template engine when it's looking for a themes templates files, and output our own one instead
 */

add_filter( 'page_template','maybe_hijack_page' );

	function maybe_hijack_page( $template ) {
		// Check the page ID and hijack if it's ID 50, this can be any conditional!
		if ( get_the_ID() == 50 ) {
			show_admin_bar( false );
			$template = dirname( __FILE__ ) . '/public.php';
		}

		return $template;
	}