ad_showthread_firstpost_start troubles with plugin

[RedFoxy]

Hi!
I created a plugin (admincp ->Plugins & Products -> Add New Plugin) with:

Product: Vbulletin
Hook Location: global_start

Plugin PHP Code:

PHP Code:

ob_start();
$variable_name = "xyzabc";
ob_end_clean(); 


If I use $variable_name in ad_header_logo and other templates it goes but if I put it in ad_showthread_firstpost_start it doesn't go.

I tried adding the variable in global too:

PHP Code:

ob_start();
global $variable_name;
$variable_name = "xyzabc";
ob_end_clean(); 


But nothing

How can I fix it?