Variable names are limited to
/[a-z0-9_]/i, which means all alpha-numeric characters plus the underscore character, they are case-sensitive.
Also, as an advice, I would recommend that you do not use any variable name starting with an underscore
_ since should I need to add a magic variable I will be using such
$_foo named variables, that way everyone avoids bad surprises.
The following are reserved variables
Retrieving scoped variables
From PHP side, you can retrieve variables scoped from template side. Here is an example:
Note: Scoped variables from included files are also in the same returned array.