I always hear that PHP is for medium and small websites whereas .NET and Java for enterprise applications. My question is about PHP. Why is PHP not a good option for enterprise web applications? Is it because if the web application becomes bigger then PHP will be slower as it is an interpreted language?

I know that corporate world will choose .NET or J2EE because of the integration with their products and because of back end services, etc. However, if we just have PHP for building sites and web applications then how can we use it to perform well with big sites?

In short, Is there a relationship between the performance of PHP and the size of the website? What are the factors that make PHP not appropriate option for big sites?