I am using PHP more than 10 years now. In the old time, PHP is a very light weighted language. This is just a interpreted language. Nowadays, many new features are introduced in PHP world, such as CSSLESS and object-oriented. They required more memory and CPU processing power. For CSSLESS, this involved a compiler too! First times compiling, that required a lot of memory. Thus, if you build a PHP website, I suggest at least you have 512MB(Well, comparing in windows side, that is not much at all).