Generate a variable usable in templates

[eisstern]

Hello,
I need a variable which i can insert into the navbar template. I want to use the plugin system of 3.6.7.
The variable should only be generated if a user is logged in. I thougth this should work:

Plugin:
hook: parse_templates

PHP Code:

if($bbuserinfo['userid'] >= 0){
  global $htSid;
  $htSid = "Dummy";  
} 


Template:

<if condition="$bbuserinfo['userid'] >= 0">
	<td class="vbmenu_control"><a href="http://domain/index.php?VAR=$htSid">My link</a></td>
</if>

The link appears in the forntend, but not the variable appended to the url. Did I something wrong in the plugin (wrong hook or bad variable declaration)?

Thanks for your help in advance