How to Extract Root Domain from Subdomains in PHP?

PHP: Extracting Root Domain from Subdomains

When working with domain names, it's often necessary to separate the root domain from any subdomains. Consider a variable containing various domain structures:

here.example.com
example.com
example.org
here.example.org

Our task is to create a function that converts all of these variations into their corresponding root domains, such as "example.com" or "example.org." Here's how:

Solution using Regular Expression

<code class="php">function get_domain($url)
{
  $pieces = parse_url($url);
  $domain = isset($pieces['host']) ? $pieces['host'] : '';
  if (preg_match('/(?P<domain>[a-z0-9][a-z0-9\-]{1,63}\.[a-z\.]{2,6})$/i', $domain, $regs)) {
    return $regs['domain'];
  }
  return false;
}</code>

Explanation:

• parse_url() breaks down the URL into its components, including the domain name.
• Regular expression extracts only the root domain using a capture group ((?P...))).

• The pattern ensures that the domain consists of:

  • Alphanumeric characters or hyphens
  • Maximum length of 63 characters for the subdomain
  • 2-6 character extension (TLD) separated by a dot

Example Usage:

<code class="php">echo get_domain("http://somedomain.co.uk"); // Outputs 'somedomain.co.uk'</code>

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