[kh99]

The problem is that the ad_showthread_firstpost_start template is processed in the function construct_postbit (in includes/class_postbit.php), so it can't see that variable you created. Putting "global $variable_name" in your plugin doesn't help because it needs to be in the function where the variable is used.

There is a hook "postbit_display_complete" right before the ad_showthread_firstpost_start is processed, so if you could make a plugin to set your variable there that should work. If not, maybe you could create a plugin that simply has "global $variable_name;" and put it on the postbit_display_complete hook.