Source

File: wp-includes/Requests/IRI.php

public static function absolutize($base, $relative) {
		if (!($relative instanceof Requests_IRI)) {
			$relative = new Requests_IRI($relative);
		}
		if (!$relative->is_valid()) {
			return false;
		}
		elseif ($relative->scheme !== null) {
			return clone $relative;
		}

		if (!($base instanceof Requests_IRI)) {
			$base = new Requests_IRI($base);
		}
		if ($base->scheme === null || !$base->is_valid()) {
			return false;
		}

		if ($relative->get_iri() !== '') {
			if ($relative->iuserinfo !== null || $relative->ihost !== null || $relative->port !== null) {
				$target = clone $relative;
				$target->scheme = $base->scheme;
			}
			else {
				$target = new Requests_IRI;
				$target->scheme = $base->scheme;
				$target->iuserinfo = $base->iuserinfo;
				$target->ihost = $base->ihost;
				$target->port = $base->port;
				if ($relative->ipath !== '') {
					if ($relative->ipath[0] === '/') {
						$target->ipath = $relative->ipath;
					}
					elseif (($base->iuserinfo !== null || $base->ihost !== null || $base->port !== null) && $base->ipath === '') {
						$target->ipath = '/' . $relative->ipath;
					}
					elseif (($last_segment = strrpos($base->ipath, '/')) !== false) {
						$target->ipath = substr($base->ipath, 0, $last_segment + 1) . $relative->ipath;
					}
					else {
						$target->ipath = $relative->ipath;
					}
					$target->ipath = $target->remove_dot_segments($target->ipath);
					$target->iquery = $relative->iquery;
				}
				else {
					$target->ipath = $base->ipath;
					if ($relative->iquery !== null) {
						$target->iquery = $relative->iquery;
					}
					elseif ($base->iquery !== null) {
						$target->iquery = $base->iquery;
					}
				}
				$target->ifragment = $relative->ifragment;
			}
		}
		else {
			$target = clone $base;
			$target->ifragment = null;
		}
		$target->scheme_normalization();
		return $target;
	}