What you are assuming is that everyone visiting your site will have HTML enabled email. Most people might, but a view might not (e.g. dtMail, pine etc).
I usually tend to quote the HTML link in the email message. A mail client like Outlook converts the link into a hyperlink anyway.
e.g.
$email="enduser@somewhere.com";
$message="Thanks for visiting our site\nFor more product information please go to http://www.your_web_site.com/products"
mail($email,"Hello",$message,"From:webmaster@your_web_site.com");