We love this language...
It is free (as in beer), mature (as in cheddar) and works (as in for me).
PHP stands for Pre-hypertext Processor. It is a widely used (very widely!) general purpose scripting language. Essentially, if one needs an HTML page to do something clever (like fetch data from a database or respond to input) PHP will do it for you.
It is a loosely typed language - so one can use variables without declaring them first (unlike C for example) and use them loosely. It supports all the 'usual suspects' in terms of language constructs and, becuse it was specially designed for web pages it has a large number of high level functions that are particularly useful when designing web sites.
It is an 'interpreted' language. This saved the compilation step when programming - saving time and lending itself to frequent iterations which (imho) produce better quality code.
It started life in 1994 as a simple set of CGI binaries written in C and has matured since then. The core development team now consists of over a hundered developers and it is in use on tens of millions (perhaps hundreds of millions) of web sites.
It is fast, secure and easy to develop in. When this means to you as a client is that we can do more in less time and so less money.
The downside? Let me know if you find one...