Introduce new exception syntax throughout the code. Example:

// definition
class Doctrine_Xyz_Exception extends Doctrine_Exception
{
    // without param
    public static function foobarIsInvalid()
    {
        return new self("Foobar is in an invalid operation. Please do something entirely different. Just do it.");
    }
    // with param
    public static function failedToFindKey($key)
    {
        return new self("I've tried and tried and tried but i can't find the key ($key) you gave me. I'm sorry. I give up.");
    }
}

// throwing code
...
throw Doctrine_Xyz_Exception::foobarIsInvalid();
or
throw Doctrine_Xyz_Exception::failedToFindKey($theKey);
...

Advantages: * Exception messages are all defined together in the Exception classes they belong to. * Exception messages can be more precise and verbose without cluttering the code that throws the exception. The code that throws the exception gets a more compact and concise look.

With namespaces the throwing code will get even more concise: throw XyzException::foobarIsInvalid();