using $ in petal

i can't figure this out

if $a is in a document, petal shows me '$a' -- good.

if $aa is in a Petal template, its stripped out.  any $ followed by 2  
or more chars is stripped out.

does anyone know why?  bug , feature , workaround ?  i was trying to  
use petal to generate some downloadable customized php snippets, and  
i've tried just about every escape sequence with no luck.