Redistribution and use in source and binary forms, with or without modification, is permitted provided that the following conditions are met:
Where the conditions are basically that you have to include a copy of the license (which applies only to the Zend part).
Seems pretty straightforward to me. (Incidentally, Zend itself uses the Zend engine in commercial closed-source applications. Of course, Zend is a special case.)
You said you were writing a "PHP extension"; that's why I was wondering why you weren't already using the copy of Zend that's there.