WHAT IS IT?
PHP stands for Hypertext Preprocessor. "But Wait!" you might say. "That makes no sense!" Well yes, I guess it technically should be HP. Of course, HP is already taken, and besides, PHP originally stood for Personal Home Page tools. It's a tool because PHP works best to create web forms, database structures, and other interactive data features on a website. So history was made, and PHP became what it is today: a scripting tool designed for web development (especially when a web page is robust enough to need to present and recall data).
WHY DO I CARE?
Ok, like me, you aren't a programmer. Fine. But maybe, like me, you're an archivist doing more than choosing tools to be used. Professionals in many fields often have the luxury of delineating what they want, how they want it, and why they want it. They'll provide the content and the ideas for the structure of a website, database, program, what-have-you, and they'll rely on IT or development staff to provide their tool. Nowadays, that luxury isn't always available due to budget cuts, low prioritization of a project, or lack of knowledgeable staff. While I am lucky enough to work with two supportive and invested IT professionals, they can't be everywhere at once. So I care because I need to be able to do a little developing and trouble shooting myself. Especially since my current tool of choice (CollectiveAccess) uses PHP. Not only do I need to know how to install it and run it, but I also need to know how to edit PHP code. But what if I wasn't using this tool? What if I was using a more out of the box program that simply ran on PHP? Well, even then, a lot of those tools (think the now deceased Archon) run on PHP. You might not be the developer, but you probably should be familiar with the jargon and the script. That way you can write compatible functions like a web form.
WHAT TO DO NOW?
Think you want to learn PHP? I know I do. In fact, I've been working on it for a while now! Below are several resources for becoming familiar with PHP. There are plenty out there, but these are the obvious favorites.
For learning: CodeCademy
For reference: W3 Schools
For Installing: ProgramOften's Tutorial
Good luck! I know I need it.