Good design dictates only writing each function once. In PHP I'm doing this by using include files (like Utils.php and Authenticate.php), with the PHP command include_once. However I haven't been able to find any standards or best practices for PHP include files. What would you at StackOverflow suggest?
I'm looking for:
Some such standards have been written already. Most large projects will follow and standard of their own.
Here is one written by Zend and is the standard used in the Zend framework. http://framework.zend.com/manual/en/coding-standard.html
Also, PEAR always had some fairly strict coding standards: http://pear.php.net/manual/en/standards.php
My preferred answer though is that for your own project you should use what you feel comfortable with, and be internally consistent. For other projects, follow their rules. The consistency allows for greatest code readability. My own standards are not the same as the PEAR ones. I do not indent with four spaces (I use tabs) and I never use camel case like function names, but nonetheless if I am editing something from another project I'll go with whatever that project does.
Whatever naming convention you end up using (I prefer to take cues from either Java or C# wherever possible) make sure if you use include files for functions that they do not actually execute any code upon including, and never include the same file twice. (use include-once or require-once)
One convention I like to use is to put each class in its own file named ClassName.class.php and then set up the autoloader to include the class files. Or sometimes I'll put them all in a classes/ subdirectory and just name them ClassName.php. Depends on how many class vs. non-class includes I'm expecting.
If you organize your utility functions into classes and make them static methods instead, you can get away with writing only a single require_once() in your top level files. This approach may or may not be appropriate for your code or coding style.
As for return types, I try to follow the conventions used in the built-in functions. Return a type appropriate to the request, or return false on failure. Just make sure you use the === operator when checking for false in the results.
The fact that you're concerned about conventions suggests you're already on the right track. If you are familiar with any other OOP language like Java, C++, C#, etc., then you'll find you can follow a lot of the same conventions thanks to the OOP goodness in PHP5.
I've done the following. First, I created an intercepting filter, to intercept all web requests, I also created a version which would work with command line commands.
Both interceptors would go to a boot strap file, which would setup an autoloader. This file as the autoloading function and a hash. For the hash the key is the class name, and the value is the file path to the class file. The autoload function will simply take the class name and run a require on the file.
A few performance tips if you need them, use single quotes in defining the file, as they're slightly faster since they're not interpreted, also use require/include, instead of their _once versions, this is guaranteed to run once, and the former is a fair bit faster.
The above is great, in fact, even with a large code base with a tonne of classes, the hash isn't that big and performance has never been a concern. And more importantly we're not married to some crazy pseudo name space class naming convention, see below.
The other option is delimited name, pseudo name space trick. This is less attractive as name spaces will come with 5.3 and I see this being gross as renaming these across the code base will be less fun. Regardless, this is how it works, assume a root for all your code. Then All classes are named based on the directory traversal required to get there, delimited by a character, such as '_', and then the class name itself, the file will be named after the class, however. This way the location of the class is encoding in the name, and the auto loader can use that. The problem with this method besides really_long_crazy_class_names_MyClass, is that there is a fair bit of processing on each call, but that might be premature optimisation, and again name spaces are coming.
eg.
/code root
ClassA ClassA.php
/subfolder
subFolder_ClassB ClassB.php