These general PHP advice might come handy for nearly every web-based project that you might be starting.
Use the latest PHP
It might seem obvious, but using the latest PHP or PHP 5.5 at least is not just about the features. Using a recent version is most importantly about the support cycle. PHP normally has a 3 years support cycle, so an older version might in time not get important security updates.
Research whether the project already exists
Of course, you might want to write everything from scratch for the pleasure of it. But if you are short of time, you might go more quickly by finding an existing project and modifying it to suit your needs. Plus the existing version might have already encountered issues and solved them, so that makes your task easier.
Use existing components
Or at least you might use Composer’s autoloader or the Symfony HttpFoundation component. Both of them would provide you with a clean and testable interface for you HTTP requests and responses. They will also enable you to code while interoperating with other projects. If you wish to access a database directly, you might want to use Doctrine DBAL. This is a backward compatible wrapper around PDO that offers the possibility of several quick shortcuts.
Use source control
Popular PHP projects can be found on GitHub or BitBucket. You might as well use these sites to share your codes, get help and submit patches to issues. Other such systems that you might find are Mercurial and Subversion.