Over the years, PHP has evolved from a script language to a programming language used in big applications with high-level architectures. As the most popular language for web applications, PHP is very fast, robust and stable by default. Coming from tiny scripts, PHP is used in large-scale web applications nowadays. In terms of business context, we need to focus on these three key factors: Scalability, Responsiveness and Resource misusage. All three factors have a high impact on hardware costs, customer loyalty and - indirectly - sales.

They mention a few ways that you can use to optimize your application's code including evaluating resource limitations, Firebug caching results and finding bottlenecks with something like XDebug.