There are several eCommerce PHP frameworks available and in our opinion it really comes down to Magento vs OpenCart. We are not anti Magento, but unless you are locked into this `system` we'd opt for OpenCart these days. Why ?
Cost and management. Most eShops really do one thing and one thing well which is to `shop` = present products in a searchable way, a single-click to buy and a simple checkout process. Much more than that and you might be turning your on-line shop into another web-application which probably merits separate or micro-sites.
There is a general consensus in the market that with Magento `anything is possible` and that's naturally true with any PHP framework, but unless you are reinventing the wheel, we don't really believe that Magento is now worth the time and the effort when far simpler systems are available and just as adaptable without the hugely over-engineered Magento framework. If you love Magento then stick to it and naturally it has worked for you.
Unless you are budgeting tens of thousands on your new eShop, we strongly recommend OpenCart as an alternative. It's so much faster to use and understand and more importantly as developers, much [much] easier to code and customise = far lower cost base for you.
Both Magento and OpenCart were borne in the mid 2000s and have both come a long way since then but Magento has frankly made their systems more complex, OpenCart less so. With so many off-the-shelf eShops available these days [and a lot of them are very restrictive and for end-users only] it is quite a big ask for an eRetailer to invest a small fortune in a framework designed for international corporations. Opencart is a mid-weight player and with the right focus and development, there is little you can't do.
A modest developer plug for OpenCart: although its documentation is a bit thin, this doesn't matter because it's a very easy framework to adapt with backend PHP and your own theme. Furthermore, unlike Wordpress which insists on all those `factory` functions, OpenCart is written in real Vanilla PHP so when you understand its structure it's really simple to customise at the CSS/JS and PHP level. We will always give YOU a custom theme - never off the shelf.
Last and not least is the extension of your shop, or extra features | add-ons. These are simple enough in OpenCart and again it's a good idea to get one off-the-shelf and then customise it. A good example of this would be Suppliers and Purchase Order Processing. You may not need these, of course, but they are all within your reach. We can do some really fancy stuff if you'd like a custom page here and there, but OpenCart is very feature-rich on the frontend these days and to make it `more` is potentially mis-guided. The backend or your `admin` is where you can really go to town.
We have no commercial arrangements or agreements with either of these platforms as they are both open-source and the views expressed here and solely those of the author.