Consistently name the $this callback variable "$that"

Consistently name the $this callback variable "$that"

Most are already, but some had differnet names. In PHP 5.4+ this is
redundant as they inherit automatically, but we need these for PHP 5.3
compatibility. Settle on "$that" to make these easier to find.

Change-Id: I0b68b2c550fde9c2da53d844421e34b8df0c42ed