No. That won't work.

The value of $pm_popup_text is what goes in the head_pm template. The value is either from the one template, or the other. There are two variables that need to be filled before the head_pm template is called...the var names, therefore, are identical in the if() half, and the else half.
Maybe I can help: what's gone wrong? Have you gone over the steps again to make sure you did them all correctly?