Diff. logo for diff. URL

[CraiovaOnLine]

So here Is my question

1 forum - 2 domains(or more)
www.domain1.com and www.domanin2.com
I would like to set a logo for each domain; when forum is accesed from domain1.com users will see logo1.gif and when from domain2.com -> logo2.gif - and so on

This is my ideea (must work with ... it's not working .. yet )
we define 2 variables in vb:

PHP Code:

$url = $_SERVER['SERVER_NAME']; 
$link = substr($url, strpos($url, '.')+1); 


after this we'll modify header section with this

<if condition="$link= 'domain1.com'">
<img src="http://www.domain1.com/logo1.gif"> 
</if>
<else /> 
<if condition="$url = 'domain2.com'">
<img src="http://www.domain2.com/logo2.gif">
</if>

So can anyone help me?
Where should i define $url and $link ?