My *development* server runs my PHP web application smoothly on PHP 5.3.8. 

However, when upgrading my PHP *production* server from PHP 5.2.6 to 5.3.8 I got several problems with this application (complaints about deprecated function `ereg_replace` and missing `date.timezone` settings) which forced me to order a rollback to 5.2.6. 

How can the same PHP code work fine on one 5.3.8 server and not on another? What are common causes for this behavior? Any suggestions?