This works and be carefull because i notice those mails with URLs are ignored by gmail and send to spam, which cause user of plugin to miss them.
p.s. of course Content-type: text/html;
I just tried and it and confirm what i possted before does work. So you did something wrong at some point.
From what i see on your code above, every field currently is using that code you posted. Please post it complete so users dont get the wrong idea. It has to be enclosed with the "if($value == 'senders_web')" like i showed you in my example. Please update the post