Getting sub domains and domain from a url

Nov 14, 2014 · 2 minute read
Category: php

At first it seems like a simple one - just use the parse_url function already built into PHP.

Unfortunately though, this just gives you the host - which is usually a combination of a subdomain - such as www - and the domain itself.

To resolve this I ended up writing the following code. It will gradually snip bits off the start of a host and then make a HTTP request until that request fails.

Once it fails, we know that we have snipped off all of the sub domains we can and we have our final domain.

Here is the code:

    public function getDomain($url)
    {
        if (!$this->_domain) {
            $host = parse_url($url, PHP_URL_HOST);
            $sanity = $this->getHeadersForUrl($host);
            if (!$sanity) {
                throw new ErrorException("No headers when sanity checking full host: $host in " . __METHOD__);
            }
            $elems = explode('.', $host);
            $subdomains = array();
            $domain = false;
            while (!$domain) {
                $check = implode('.', $elems);
                $headers = $this->getHeadersForUrl($check);
                if ($headers) {
                    $subdomains[] = array_shift($elems);
                } else {
                    array_unshift($elems, array_pop($subdomains));
                    $domain = implode('.', $elems);
                }
            }
            $this->_domain = $domain;
        }
        return $this->_domain;
    }

    public function getHeadersForUrl($url, $followOnLocation = true)
    {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_HEADER, true);
        curl_setopt($ch, CURLOPT_NOBODY, true);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_FOLLOWLOCATION, $followOnLocation);
        curl_setopt($ch, CURLOPT_MAXREDIRS, 10);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);

        $data = curl_exec($ch);

        return $data;
    }