Need to have http added if not present before www - php

Question

My function gets all the facebook description text from our string $FBdescription. This preg_replace does everything I need except when it finds a url like www.myurl.com in the description it places that in the href= and because it does not contain the http:// at the begining it of course causes problems when you click the link. How can this be adjusted to append the http:// if it's not there.
function fts_facebook_tag_filter($FBdescription)
//Converts URLs to Links in our Description Text
$FBdescription = preg_replace('#(?!(?!.*?<a)[^<]*<\/a>)(?:(?:https?|ftp|file)://|www\.|ftp\.)[-A-‌​Z0-9+&#/%=~_|$?!:,.]*[A-Z0-9+&#/%=~_|$]#i', '\0', $FBdescription);
return $FBdescription;
}

Answer 1

Not best but working solution: (prepends http:// if missing, inside href attribute only)
$FBdescription = 'BZRK Records and BZRK Black label Artists playing here More info : www.bzrk.agency https://www.facebook.com/daphne.merks/posts/988562257858538';
//Converts URLs to Links
$FBdescription = preg_replace('#(?!(?!.*?<a)[^<]*<\/a>)(?:(?:https?|ftp|file)://|www\.|ftp\.)[-A-‌​Z0-9+&#/%=~_|$?!:,.]*[A-Z0-9+&#/%=~_|$]#i', '\0', $FBdescription);
$splitano = explode("www", $FBdescription);
$count = count($splitano);
$returnValue = "";
for($i=0; $i<$count; $i++) {
if (substr($splitano[$i], -6, 5) == "href=") {
$returnValue .= $splitano[$i] . "http://www";
}
else if($i < $count - 1){
$returnValue .= $splitano[$i] . "www";
}
else {
$returnValue .= $splitano[$i];
}
}
echo $returnValue;