url - PHP: Get Domain (without subdomain) of any Address available as String -

recently question has been asked, how domain of url available string.

unfortunately question has been closed, , far linked answers pointed solutions using regex (which fails special cases .co.uk) , static solutions, considering exceptions (which ofc. might change on time).

so, searching generic solution question, work @ time , found one. (at least couple of tests positive)

if find domain attempted solution not work, feel free mention it, , i'll try imrpove snipped cover case well.

to find domain of string given, three-step solution seems work best:

• first, actual hostname, using parse_url (http://php.net/manual/en/function.parse-url.php)
• second, query dns-server "top-most" a-record available. (i used checkdnsrr purpose: http://php.net/manual/en/function.checkdnsrr.php)
• last not least: perform validations make sure not running "default response".

i performed tests , seems result expected. method directly generates output, can modified return domain name instead of generating output:

<?php  getdomain("http://www.stackoverflow.com"); getdomain("http://www.google.co.uk"); getdomain("http://books.google.co.uk"); getdomain("http://a.b.c.google.co.uk"); getdomain("http://www.nominet.org.uk/intelligence/statistics/registration/"); getdomain("http://invalid.fail.pooo"); getdomain("http://anotheronethatshouldfail.com");   function getdomain($url){   echo "searching domain '".$url."': ";   //step 1: actual hostname   $url = parse_url($url);   $actualhostname = $url["host"];    //step 2: top-down approach: check dns records first valid a-record.   //re-assemble url step-by-step, i.e. www.google.co.uk, check:    // - uk   // - co.uk   // - google.co.uk (will match here)   // - www.google.co.uk (will skipped)    $domainparts = explode(".", $actualhostname);   ($i= count($domainparts)-1; $i>=0; $i--){     $domain = "";     $currentcountry = null;     ($j = count($domainparts)-1; $j>=$i; $j--){       $domain = $domainparts[$j] . "." . $domain;        if ($currentcountry == null){         $currentcountry = $domainparts[$j];       }     }     $domain = trim($domain, ".");     $validrecord = checkdnsrr($domain, "a"); //looking class records      if ($validrecord){        //if host can resolved ip, seems valid.        //if hostname returned, invalid.          $hostip = gethostbyname($domain);          $validrecord &= ($hostip != $domain);         if ($validrecord){          //last check: dns server might answer 1 of isps default server ips invalid domains.          //perform test on querying domain of same "country" invalid sure obtain          //ip list of isps default servers. compare response of current $domain.          $validrecord &= !(in_array($hostip, gethostbynamel("iiiiiiiiiiiiiiiiiinvaliddomain." . $currentcountry)));        }     }      //valid record?     if ($validrecord){       //return $domain;       echo $domain."<br />";       return;     }   }   //return null;   echo " not resolved.<br />"; }   ?> 

output of example above:

searching domain 'http://www.stackoverflow.com': stackoverflow.com searching domain 'http://www.google.co.uk': google.co.uk searching domain 'http://books.google.co.uk': google.co.uk searching domain 'http://a.b.c.google.co.uk': google.co.uk searching domain 'http://www.nominet.org.uk/intelligence/statistics/registration/': nominet.org.uk searching domain 'http://invalid.fail.pooo': not resolved. searching domain 'http://anotheronethatshouldfail.com': not resolved. 

this limited set of test-cases cannot imagine case, domain has no a-record.

as nice side-effect, validates urls , not rely on theoretically valid formats last examples showing.

best, dognose