Pin XDebug to version 2.2.7, the last one that supported PHP 5.3.
Latest version of XDebug (2.3.2 as of this writing) requires PHP 5.4, but this dev environment uses 5.3.10-1ubuntu3.18.
From announcements on http://xdebug.org/
[2015-02-22] - Xdebug 2.3.0 is out!
...
This release drops support for PHP 5.2 and PHP 5.3, and raises the default max nesting level to 256. It also fixes a bunch of issues as found in Xdebug 2.2.7.
@matt_attaway
