What's 'easiest' ecommerce platform to use and manipulate?

OK, so there are quite a few packages out there

Magento, Prestashop, Opencart, OSCommerce

But... I wanted some opinions on which was the 'easiest':

- to get your head round and figure how things work!

- to adapt and change - from simple things like themes to functionality

ALSO:

- which is fastest? I love Wordpress, but someone told me it's big fat elephant when running PHP code because of all the extras that come lumbered with it

Dumb question: if I'm trying to learn PHP, would it be advisable to try and work out how one of these system works entirely? I mean literally go through every single file! There could be 1000's! But I would have assumed there would only be 20 - 30 files that we relevant to the main core? Or is that just a big bad idea?

Just looking for pointers/opinions

Thanks

OM

if I'm trying to learn PHP, would it be advisable to try and work out how one of these system works entirely?

Not at all. There is allot of terrible code around. Who is to say that these code bases are the best quality? To be honest, I've not looked at them, but Iv'e not heard any good things about them either.

IMO, you would be much better off learning from a well written framework. Zend comes to mind.

interesting comments

i've started by going through a few videos teaching OOP - that was good because it also showed some of the tools you should use

then i've been though a video going through a basic shopping cart

i've also got the oreailly php/javascript/mysql book - i'm planning to read this thoroughly - should take that long as i don't need to learn basics like operators and other simple things

would u say, i should start looking into the frameworks as well?

thanks