PHPOffice/PHPExcel — pull request #60 by Synchro (2012-10-18 14:12:13+00:00)
Title: Change use of is_a() to use instanceof

While is_a() is no longer deprecated, there are bound to be some people using PHP versions that still have it marked as deprecated.
This is the only occurrence of is_a in the whole of PHPExcel; everywhere else it's using instanceof, so I figured it would be sensible to make it consistent.