PHP is a widely-used general-purpose server-side scripting language that is especially suited for dynamic web page development and can be directly embedded into HTML documents. PHP and HTML can co-exist seemlessly in the same document without trouble. PHP has a syntax and structure not unlike C++ and several other programming languages. In fact, a little knowledge of programming in almost any other language will make the transition to PHP much easier for the new coder.
The main PHP website (link above) has an introductory tutorial and an online manual with a downloadable, printer-friendly, version as well.