How to give a field a default value

[ZiNgA BuRgA]

Okay, so you want a Blank Replacement Value, not a Default Value.
In which case, you place the code there.

As for your PHP, you need to globalise your $sub and $build variables, otherwise nothing outside the template can actually access them.
You may wish to review your code though.  There's certainly stuff in there that you probably aren't intending, and the logic seems unnecessarily complicated.