why do i have to type full URL to run PHP code example

Example 1: php get full url

$fullURL = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];

Example 2: get full url php

$actual_link = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? "https" : "http") . "://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";