We are planning to do a large application, and we are tented to do it in php and mysql, because of it cost, but this application might grow a lot, and the database engine could have thousands of connections per second, so I am not sure what technology could handle this volume better. I would like to know what technology I should use, and why I should use it. Many thanks for your help guys!!!!
Regards
Both php and MySQL are able to scale and there are various large websites that are built on this stack. This is less a question of technology but of the application of technology, i.e. with an appropriate architecture you should be good with both of your suggested technology stacks.
I've worked on some VERY large projects that involve CMS and online registration systems written in PHP/MySQL that handle 1million+ per day. MySql is very powerful, even more so now that it has the knowledge of Oracle behind it. I've been involved with enterprise-level sites will tens of millions of records handled by effective MySQL clusters. In fact, it (php) powers one of the most trafficked sites on the web, Facebook. And it's all free....
Here's a great illustration: I was hired as webmaster at one Olympic NGB, having just left another. Both were in the process of building new websites, both relatively similar in size and function. The one I worked at opted to develop in-house using PHP/MySQL, the one I had left opted to outsource in .net/SQL. My project came in with just minimal consulting fees, hosted on two web servers and a db server that we built ourselves. The .net option utilized 15 servers when all was said and done (production and testing) with a development price tag over $1million. It was even spotlighted by Microsoft as a case study for its "success" Today, both are still going. The PHP/MySQL site has been continually refined by the same staff, while the .net site has required additional significant investment in more outside consultants, software, and hardware for upgrades, improvements, and maintenance.
Moral of the story: leverage your existing knowledge. Research your options. Don't give into hype, especially when it comes from those who stand to make a lot of money. Document like crazy.
As I have answered elsewhere, your first question should be "Where am I going to deploy this application?" If you're building a large infranet application for deployment in a Windows environment, with user authentication handled by Active Directory, then you're probably better off with ASP.NET / SQL Server. If you're going to be deploying this as an extranet (or cloud) application then you'll find better portability with PHP / MySQL.
If it's something in between, then you'll need to leverage cost, scalability, portability, and ease of development, and no one else will be able to give you the right answer, only guesses. In that case, you're going to need to do some prototyping for testing performance and scalability, doing work that you expect your application to do.