eBay Developer Program's Adam Trachtenberg talks about PHP and the eBay API

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For hosts actually contemplating the move to PHP5, what do you find is the hardest part of the move?
There are only 2 things that are tricky and you really need to watch out for. First, if you are making extensive use of Object Oriented features in PHP4 there are things that are different. Things like the constructor of the class is now __construct() whereas it used to be a method named the same name as the class. But PHP5 does some fallback things to ease that along. Also “passed by reference” verses “passed by value”. In PHP4 we had to put a lot of ampersands together to get things passed by reference. If you are doing a lot of OO code then it’s something to look out for but if you are mainly working in procedural it’s not really an issue.

The other big issue that is tricky is that all the XML extensions in PHP5 have been re-written. They are much better and they have been standardized both within PHP but also they use a lot of the XML standards like DOM. Because PHP5 enabled us to do some things that PHP4 didn’t allow, they are also now more robust. If you are using the older XML extensions in PHP4, those extensions were not ported to PHP5. If you have code that relies on the old XSLT, the Sablatron library or the old DOMXML extension then you’ll have to fix your code to work with the new extensions. The book touches on this issue as well. It talks about the features that went away and the incompatibilities...

full article: http://devzone.zend.com/node/view/id/781