I've not been coding for all that long and wish to know what else to learn from the more experienced coders, in order of priority.

Some things I've already learned

-Class use.
-Regex
-MySQL/SQLLite/MSSQL
-File Handeling

I didn't learn complex regex until later on. I've had a grasp of basic regex from about 3 months into me learning to program, though I always used to use the soon to be obsolete ereg* functions. As a side-note, I think PHP should use regex a little similar to the way PERL does it. Perhaps something like the following:


But I digress MySQL, SQLite and MSSQL are all very similar and I'm sure you know the basics to keep you ticking along nicely. File handling is a bit of a shallow topic really. Therefore, I'd say learn about classes! As that's the way programming is really going these days. OOP is the new buzz acronym, and so knowing that well will, I feel, set you in good stead for the future.

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Thank Wild, I'll be sure to make a note to read more articles on the theory.

I agree with Wildhoney, OOP, Classes, design patterns, all good things to be looking at. On a simaler track, take a look at some PHP frameworks such as CakePHP or the Zend Framework - all use the MVC pattern and make heavy use of OOP and design pattern principles so you'll learn plenty there.

Alan

How about:

• XML handling
  SimpleXML, DOM, XPath, XSLT, etc.. A simple introduction would perhaps be to first parse, then modify, then create, an RSS feed since there are tonnes of tutorials on that subject if you get stuck. For the purposes of learning, don't just achieve a task in one way (e.g. SimpleXML parsing an RSS feed) but delve into different approaches for completing the task (e.g. Use DOM instead). But I digress.
• Handling different output types
  This is, in the long term, likely something you'll use without thinking much about it. Say you have a script doing something, would it be too much to ask to have the results available in HTML, plain text, an image, XML, JSON (JavaScript Object Notation)? This is more of a concept, a way of thinking, than learning any specific functions or methods.
• Image generation
  Lets face it, we all love making funny little images with PHP code. You should as well (if you don't already).
• Read through the PHP Manual
  It's long, and there's likely a lot of stuff you'll probably never use, but if you have the patience (or tenacity!) to sit down and read, you'll command much greater knowledge than fumbling through over time. You're allowed to skip function libraries that don't look interesting.

Note: They're not in order of priority by any means, nor are they necessarily required learning. Just a few suggestions that you might, perhaps, like to look into.