str_replace member field

[Bilderback]

I've had all sorts of problems trying to convert a members profile field to URL.
My example is a MySpace field which allows a member to enter their account ID.
What I am trying to turn the field entry into a hyperlink shown in the users profile page.
Example:
User enters MySpace ID of 123456
I want to create a plugin to replace the field so when viewing their profile, it shows as:
http://www.myspace.com/123456

Below is what I have been trying but I am definitely going wrong somewhere.
Template: profile_myspace
MySpace Field: 5
Hook: member_customfields

global $vbulletin;
if ($vbulletin->options['profilefieldid==5'])
{
  eval('$profilefield[field5] = "' . fetch_template('profile_myspace') . '";');
}

Can anyone point me in the right direction?
Thanks