PHP Resolving URL format with Base URL Relative Path into Absolute

Transform Relative Path Into Absolute URL Using PHP

function rel2abs($rel, $base) {
    /* return if already absolute URL */
    if (parse_url($rel, PHP_URL_SCHEME) != '') return $rel;

    /* queries */
    if ($rel[0] == '?') return explode("?", $base)[0] . $rel;

    /* anchors */
    if ($rel[0] == '#') return explode("#", $base)[0] . $rel;

    /* parse base URL and convert to local variables: $scheme, $host, $path */
    extract(parse_url($base));

    /* Url begins with // */
    if ($rel[0] == '/' && $rel[1] == '/') {
        return "$scheme:$rel";
    }

    /* remove non-directory element from path */
    $path = preg_replace('#/[^/]*$#', '', $path);

    /* destroy path if relative url points to root */
    if ($rel[0] == '/') $path = '';

    /* dirty absolute URL */
    $abs = "$host$path/$rel";

    /* replace '//' or '/./' or '/foo/../' with '/' */
    $re = array('#(/\.?/)#', '#/(?!\.\.)[^/]+/\.\./#');
    for ($n = 1; $n > 0; $abs = preg_replace($re, '/', $abs, -1, $n)) {}

    /* absolute URL is ready! */
    return "$scheme://$abs";
}

Testing ...

echo '<h4>Queries</h4>';
echo rel2abs("?query=1", "http://something.net/path/test.php");
echo '<br>';
echo rel2abs("?query=1", "http://something.net/path/test.php?old_query=1");

echo '<h4>Anchors</h4>';
echo rel2abs("#newAnchores", "http://something.net/path/test.php?a=1");
echo '<br>';
echo rel2abs("#newAnchores", "http://something.net/path/test.php?a=1#something");

echo '<h4>Path</h4>';
echo rel2abs("/testother.php", "http://something.net/folder1/folder2/folder3/test.php");
echo '<br>';
echo rel2abs("./../../testother.php", "http://something.net/folder1/folder2/folder3/test.php");
echo '<br>';
echo rel2abs("./../testother.php", "http://something.net/folder1/folder2/folder3/test.php");
echo '<br>';
echo rel2abs("./testother.php", "http://something.net/folder1/folder2/folder3/test.php");
echo '<br>';
echo rel2abs("testother.php", "http://something.net/folder1/folder2/folder3/test.php");

echo '<h4>Url begins with //</h4>';
echo rel2abs("//google.com/path/", "https://something.net/path/test.php");
echo '<br>';
echo rel2abs("//google.com/path/", "http://something.net/path/test.php");

Test Output ...

Queries

http://something.net/path/test.php?query=1
http://something.net/path/test.php?query=1

Anchors

http://something.net/path/test.php?a=1#newAnchores
http://something.net/path/test.php?a=1#newAnchores

Path

http://something.net/testother.php
http://something.net/folder1/testother.php
http://something.net/folder1/folder2/testother.php
http://something.net/folder1/folder2/folder3/testother.php
http://something.net/folder1/folder2/folder3/testother.php

Url begins with //

https://google.com/path/
http://google.com/path/