Fix indent() and unindent()

`indent()` and `unindent()` were ignored since 2005. They are referencing non existing field name.

This issue is from:

https://github.com/pornel/PHPTAL/commit/6e48d2d489b41c8034331887afb759dcf758bbe4#L51L112

At that time PHP was enough fuzzy but today's PHP is becoming more strict. Also I guess that indented PHP code generation will be helpful.