Can PHP/mySQL handle 1,000,000 unique users onlinie at the same time? Or should we use a more powerful system like Zend? Or does it depend more on the web hosting systems like load balancing, etc?
If you have 1 million simultaenous users, that means you'd need to be getting at least 300 million visitors a day (and close to a billion pages per day)*. When you're in those numbers, then you're Facebook or Google and at that point, it doesn't matter what language or framework you used to begin with, because you're pretty much running on your own platform by the time you get that big.
Remember, Facebook started off with PHP and as they scaled up, eventually they ended up writing a compiler to translate PHP into C++ code, and compile that to run on the server.
However, when you're just starting out, your first priority is to become popular: you don't become popular overnight and you'll have plenty of time to adapt your design to scale with your popularity.
* Just back-of-an-envelope calculation...
In theory, PHP and MYSQL will handle an infinite connections query, it's more depending on your global Hardware infrastructure than on your software.
If you got a lot of HI-Perf servers with a high speed connection backbone, you'll be able to handle a lot of traffic, but, for my own experience, if you've got a website wich handle around 1Million of unique users connections, you should working on google or some other big company.
On my website, we've got an average of 15000 unique connexions with a peek at 25000 during holidays time.