Check the other SPL Exception classes and extend one of those if your intended exception is a subclass of one of those. This allows more finesse when catching.

As previously noted exception linking was recently added (and what a god-send it is, it certainly makes layer abstraction (and, by association, exception tracking) easier).

Since <5.3 was lacking this useful feature I took some initiative and creating a custom exception class that all of my exceptions inherit from:

<?php

class SystemException extends Exception
{
    private $previous;
    
    public function __construct($message, $code = 0, Exception $previous = null)
    {
        parent::__construct($message, $code);
        
        if (!is_null($previous))
        {
            $this -> previous = $previous;
        }
    }
    
    public function getPrevious()
    {
        return $this -> previous;
    }
}

?>

Hope you find it useful.

Support for exception linking was added in PHP 5.3.0. The getPrevious() method and the $previous argument to the constructor are not available on any built-in exceptions in older versions of PHP.

Custom exception classes can allow you to write tests that prove your exceptions
are meaningful. Usually testing exceptions, you either assert the message equals
something in which case you can't change the message format without refactoring,
or not make any assertions at all in which case you can get misleading messages
later down the line. Especially if your $e->getMessage is something complicated
like a var_dump'ed context array.

The solution is to abstract the error information from the Exception class into
properties that can be tested everywhere except the one test for your formatting.

<?php

class TestableException extends Exception {

        private $property;

        function __construct($property) {

                $this->property = $property;
                parent::__construct($this->format($property));

        }

        function format($property) {
                return "I have formatted: " . $property . "!!";
        }

        function getProperty() {
                return $this->property;
        }

}

function testSomethingThrowsTestableException() {
        try {
                throw new TestableException('Property');
        } Catch (TestableException $e) {
                $this->assertEquals('Property', $e->getProperty());
        }
}

function testExceptionFormattingOnlyOnce() {
        $e = new TestableException;
        $this->assertEquals('I have formatted: properly for the only required test!!',
                $e->format('properly for the only required test')
        );
}

?>

It's important to note that subclasses of the Exception class will be caught by the default Exception handler

<?php
    
    /**
     * NewException
     * Extends the Exception class so that the $message parameter is now mendatory.
     * 
     */
    class NewException extends Exception {
        //$message is now not optional, just for the extension.
        public function __construct($message, $code = 0, Exception $previous = null) {
            parent::__construct($message, $code, $previous);
        }
    }
    
    /**
     * TestException
     * Tests and throws Exceptions.
     */
    class TestException {
        const NONE = 0;
        const NORMAL = 1;
        const CUSTOM = 2;
        public function __construct($type = self::NONE) {
            switch ($type) {
                case 1: 
                    throw new Exception('Normal Exception');
                    break;
                case 2:
                    throw new NewException('Custom Exception');
                    break;
                default:
                    return 0; //No exception is thrown.
            }
        }
    }
    
    try {
        $t = new TestException(TestException::CUSTOM);
    }
    catch (Exception $e) {
        print_r($e); //Exception Caught
    }
    
?>

Note that if an Exception is caught once, it won't be caught again (even for a more specific handler).