URL prefix to include one PHP page

Try get_template_part().

For example, in your index.php template file:

/**
 * Index template file
 */

/**
 * Include header
 */
get_header();

/**
 * Include arbitrary file
 */
get_template_part( 'foobar' );

This will include a template-part file, foobar.php, located in the Theme’s root directory.