PHP function to automatically get a host url from $_SERVER variables to enable creation of absolute links etc. (Apache)

get_host_url.php

<?php
//e.g.semi-colon separated define list of all allowable hosts (does not need to show sub-domains - works automatically)
//$valid_hosts = 'first-host.com;second-host.com';

//create absolute url for page address - quite tricky to make this work on linux / windows and shared hosting and also
//php built-in web server.
//Constructed URL ends up in a trailing slash with the $target parameter appended.
//defaults to not checking a valid host list unless one is supplied
function get_host_url($target = '', $valid_hosts = '*'){
    // Get HTTP/HTTPS (the possible values for this vary from server to server)
    //$scheme = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] && !in_array(strtolower($_SERVER['HTTPS']),array('off','no'))) ? 'https' : 'http';
    //handle forwarded load-balancer cases too
    $isSecure = false;
    if (isset($_SERVER['HTTPS']) && (strtolower($_SERVER['HTTPS']) == 'on' || $_SERVER['HTTPS'] == '1')) {
        $isSecure = true;
    }elseif(!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' || 
            !empty($_SERVER['HTTP_X_FORWARDED_SSL']) && $_SERVER['HTTP_X_FORWARDED_SSL'] == 'on') {
        $isSecure = true;
    }
    $scheme = $isSecure ? 'https' : 'http';        

    if (!in_array(intval(isset($_SERVER['SERVER_PORT']) ? $_SERVER['SERVER_PORT'] : 80), [80, 443])) {
        $port = ":".intval($_SERVER['SERVER_PORT']);
    } else {
        $port = '';
    }

    $valid_hosts = preg_split('/;/',$valid_hosts, -1, PREG_SPLIT_NO_EMPTY);
    $host = "";
    foreach($valid_hosts as $h){
        if($h === '*' || preg_match('#'.preg_quote($h,'#').'(|:\d+)$#', $_SERVER['HTTP_HOST'])){
            $host = filter_var($_SERVER['HTTP_HOST'], FILTER_SANITIZE_URL); //can't use filter_input dur to fastcgi bug
            $host = preg_replace('#:\d+$#','', $host); //remove port
            break;
        } 
    }

    //ensure trailing slash and handle windows "\" oddities
    $path = rtrim(strtr(dirname(filter_var($_SERVER['PHP_SELF'], FILTER_SANITIZE_URL)),'\\','/'),'/').'/';

    return $scheme . "://" . $host . $port . $path . $target;        
}
/* end */