Description:
------------
spl_autoload_register() builds the autoload stack based upon the 
sequence in which methods are added to the stack.  This gives precedence 
to the first methods loaded into the stack, not the subsequent ones.  
This can be an issue when someone creates an initial autoloader to cover 
the most generic of situations and is designed to error out when nothing 
can be found, either by using require() or by throwing an exception.  If 
I wanted to add a secondary autoloader to account for an additional set 
of classes I would need to either unregister the initial autoloader and 
account for those scenarios or change the order of the autoload stack.

I realize this suggest an API change, which is obviously less than 
desirable.  Perhaps, though, a method like spl_autoload_preregister() 
could be added to prepend autoloaders to the stack, rather than append, 
thus giving them precedence in the call order.

Reproduce code:
---------------
function genericAutoload($class) {
    if (!include($class . '.php')) {
        throw new Exception("Can't include!");
    } else {
        return true;
    }
}

function secondaryAutoload($class) {
    include('library/' . $class . '.php');
}

spl_autoload_register('genericAutoload');
spl_autoload_register('secondaryAutoload');


Expected result:
----------------
Autoloaders added later on in the code would be called first, so the 
stack would be called in the reverse order it is now, or new autoloaders 
would be prepended to the autoloader stack.  This would allow the first 
initial autoloader to error out as the last catch-all, but subsequent 
autoloaders to accomodate particular scenarios, where they would most 
likely not produce a total failure.  The reproduce code will show an 
instance where the secondary autoloader will never be called because the 
first autoloaderis called and designed to fail if a file cannot be found 
for the given class.