"While you can use PHP to create command-line scripts for such tasks as systems administration and traditional data processing, the language predominantly powers Web applications. In this employment, each PHP application resides on a server and is invoked through a proxy, such as Apache, to process an incoming request. With each request, a typical PHP Web application runs very briefly, yielding a Web page or an XML data structure.
"Given the brevity of execution and the tiered construction of Web applications--there's the client, the network, the HTTP server, the application code, and the underlying database--it can be difficult to isolate bugs in PHP code. Even if you assume that all the tiers except the PHP code work flawlessly, tracking down an error in PHP code can be frustrating, especially (and perhaps ironically) as an application leverages more and more classes..."
