PEAR Coding Standards
The documents in PCS outline a structured way in which code should be written in order for a package to be accepted as part of the PEAR project. The standards are quite detailed and contain mostly points of style, such as identifier naming conventions and a consistent style to use when declaring functions and classes.
This may sound a little daunting, but as your scripts become more complicated, you will realize how important it is to write readable code, and you will begin to develop a clear coding style. The PCS documentation simply formalizes a set of guidelines for writing readable PHP.
You can find the PCS documents online at http://pear.php.net/manual/en/standards.php.