As of version 5.0, PHP has had the ability to dynamically include required classes as needed - without requiring the developer to manually include all possible dependencies beforehand. This means that in cases where your code execution never touches 39 of the 40 classes in the project, it loads, parses, and runs that much faster.
There is a performance hit for actually having to call the __autoload() method, but if you're in a situation where the hit for executing a few extra comparison calls is unacceptable... you probably aren't developing in PHP in the first place 
Almost all of the php I've written in the last 2-3 years uses autoloading, and it has probably saved me hundreds of hours of aggravation.
In most of my projects, the first line of any script or class usually looks something like this:
require_once "/var/www/common/lib.php";
Then lib.php usually reads something like this:
<?
function __autoload( $class ) {
include_once( "$class.php" );
}
?>
And that is all that is strictly required to make the magic happen. It is fast, it is easy to understand, it is easy to use. You can use require_once() or include_once() and there is very little meaningful difference.
I've looked around the net and found several other attempts at improving on this simple mechanism. But they invariably overcomplicate things. They attempt to recurse source directories, cache filename->class differences to the filesystem, and otherwise turn what should be a simple filesystem operation that the php environment supports natively into a mess of exception handling and wheel reinvention.
There are obviously theoretical instances where you might want to have more than the one require_once/include_once line... but I've honestly never encountered one myself.
I mean, you could try to throw an exception if the file didn't exist or otherwise failed to load... but nothing will happen. Failure to instantiate a nonexistant class is a fatal error in PHP, and will be handled as such with or without you - preempting any attempt at throwing an exception.
The only thing you can add is a bit of extra diagnostics or maybe logging to a separate location.
Assume that we have a file 'test.php':
<?
require_once "autoload.php";
$frog = new Frog();
?>
If autoload.php contains a simple simple autoload function that uses require_once(), and Frog.php doesn't exist anywhere in your include path, the results will look something like this:
ammon@kif:~$ php test.
php
Warning: require_once(Frog.php): failed to open stream: No such file or directory in /home/ammon/autoload.php on line 3
Fatal error: require_once(): Failed opening required 'Frog.php' (include_path='.:/usr/share/php:/usr/share/pear') in /home/ammon/autoload.php on line 3
If we had used an include_once() call, the output is similar, but slightly more informative:
ammon@kif:~$ php test.
php
Warning: include_once(Frog.php): failed to open stream: No such file or directory in /home/ammon/autoload.php on line 3
Warning: include_once(): Failed opening 'Frog.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in /home/ammon/autoload.php on line 3
Fatal error: Class 'Frog' not found in /home/ammon/test.php on line 4
So that's probably a bit more useful in tracking down the error. Require calls don't return anything - they throw a fatal error on failure. Include calls, however, return FALSE on failure and TRUE if the file is (or, in the case of include_once, has already been) successfully included. So you can include_once() and write to a separate logfile (or to the output stream...) if you need more information than the fatal error already provides you.
<rant>
To those who insist on giving your classes and their containing files different names... umm. Wow.
If I have a class called DatabaseConnection, I'm going to put it in a file called DatabaseConnection.php. If I'm working with strange people who somehow don't think that is explicit enough, I might call it DatabaseConnection.class.php and tweak the autoload method ever so slightly to compensate. There's no good reason to put it in a file called projx-database_connection.incl or something. No. There isn't.
If you want to organize your classes into a meaningful directory structure... good for you. Use PHP's built-in include_path ini option. Don't waste time trying to cascade down a directory structure searching for the classes - just make sure your includes are all in a set of reliable locations. You don't actually have to edit the php.ini file and bounce Apache or your php-cgi processes, just define the additional include paths in the same file where you define your autoloader:
set_include_path(
get_include_path() . PATH_SEPARATOR .
"/var/www/includes" . PATH_SEPARATOR .
"/var/www/includes/apple" . PATH_SEPARATOR .
"/var/www/includes/banana"
);
Naturally, you could turn that into some function calls to dynamically register and unregister directories, etc... but at that point, you're probably hurting yourself again. If your codebase is being reorganized enough to make maintenance of the list of include dirs onerous without full time intervention, something else has probably already gone very wrong. At best, the code probably doesn't work anyway, so any brief delay in updating the list can't hurt any more than whatever else is happening.
</rant>
But seriously. __autoload() is your friend. It will help clean up your code if you let it. It can help enforce naming conventions. It can even improve performance... so long as you refrain from using it to shoot yourself in the foot.
