Here's a good 2 part series called Develop rock-solid code in PHP: Use variables effectively.

Part-1[,phb01=PHBvariables1 ] provides good information about factors that must be considered during planning, designing, and even writing code.

The second artice[,phb01=PHBvariables2 ], just released, shows you how to use variables effectively. It also shows how to make script configuration easy by constructing a configuration file parser using variable variable names in PHP.