How I add an extra part to this php code so it sends info from another field called

[Joe]

"phone" in an email? // check to see if verificaton code was correct
if(md5($verif_box).'a4xn' == $_COOKIE['tntcon']){
// if verification code was correct send the message and show this page
mail("[email protected]", 'Online Form: '.$subject, $_SERVER['REMOTE_ADDR']."\n\n".$message, "From: $from");
// delete the cookie so it cannot sent again by refreshing this page
setcookie('tntcon','');
} else {
// if verification code was incorrect then return to contact page and show error
header("Location:".$_SERVER['HTTP_REFERER']."?subject=$subject&from=$from&message=$message&phone=$phone&wrong_code=true");
exit;