While I agree with Paul's suggestion, here is something else that may also help:
Figure out what kind of stuff you haven't done yet with the language you are using. Then do it: you will learn a lot on the process, and once it is done you have something you can add to your resume.
For example, do most of your projects share the same boilerplate? (Mine use to). It's fine to have a solid way to approach common problems, but try to find a project where that "template" doesn't work, and go for it.
About what to read, http://www.php.net/manual/ is the most obvious place to go. Keep it handy (on your bookmarks or, even better, as a custom search service on your browser), and look there first everytime you're dealing with something new or unknown.
Before you are suited to lead a team, you should be capable of looking at a problem and figuring out the rough path you'd have to walk to solve it (besides other skills, such as communication, organization, etc). The only reliable and efficient way to earn that ability is to acquire experience in a wide range of different problems. Here are some examples of things you may try to do if you haven't done anything like it before:
- AJAX: It involves a significant part of server-side (PHP) programming; but if you can also handle the client part, you'll get a broader perspective of what it can do (and hence which problems can it solve).
- XML instead of DB: For example, build a minisite that replaces each DB table with an XML site (don't bother too much if things are slower, XML access isn't as optimized as DB storage engines). It may be interesting to use both XML-specific APIs and raw text-processing APIs. As an extra excercise, you might consider a project where XML is stored inside a DB (but significant manipulation of the XML itself should take place for the excercise to be useful).
- Use tools that are not meant for the job, for the job: For example, try to tweak some wiki software to build a forum site, or build some kind of game out of a CMS or BB system (the game doesn't need to be fun, but it should be complex enough to require some tweaking of the underlying software). This will boost your skills in several ways:
- 1 it will teach you how to work with unusual APIs (which means that you will be much more able to work with any new API as long as you have some basic docs about it).
- 2 it will teach you a criticall skill for team-working: code-sharing: you will be tweaking code that was written by someone else, and probably you'll end up hating that code. Thanks to this, you will become able to identify parts of code that you write and, while they may seem obvious to you, may be really obscure for third-parties.
- 3 Also, since you may be delving into code written by multiple people, you will delve into many idioms, techniques, conventions, etc; which are always good to know about.
- Non-standard web-based apps: try to build something on PHP that doesn't work directly on browsers, but on a custom desktop application made expressely for it. This is also a chance to learn some desktop programming language; and also you will discover that, sometimes, making apps based on different technologies communicate with each other can become really tricky.
- And anything you can come up with! These are just ideas, the broader you explore, the further your skills will reach.
Hope this helps.