Apparently Zend made Dmitry an offer he couldn't refuse :)
From chat log of July 2nd, 2003:
<@Zeev> Turck_ is copying every darn thing that we're doing <@Zeev> I'm really not sure why this Dmitri guy is doing it <@Derick> Zeev: ever asked him?
On firstname.lastname@example.org yesterday:
From: Dmitry Stogov <dmitry - zend - com> To: email@example.com Subject: [PHP-DEV] CVS Account Request: dmitry Overall & engine development