A User defined Exception class can be defined by extending the built-in Exception class. The members and properties below, show what is accessible within the child class that derives from the built-in Exception class.
Example #1 The Built in Exception class
<?php
class Exception
{
    protected $message = 'Unknown exception';   // exception message
    private   $string;                          // __toString cache
    protected $code = 0;                        // user defined exception code
    protected $file;                            // source filename of exception
    protected $line;                            // source line of exception
    private   $trace;                           // backtrace
    private   $previous;                        // previous exception if nested exception
    public function __construct($message = null, $code = 0, Exception $previous = null);
    final private function __clone();           // Inhibits cloning of exceptions.
    final public  function getMessage();        // message of exception
    final public  function getCode();           // code of exception
    final public  function getFile();           // source filename
    final public  function getLine();           // source line
    final public  function getTrace();          // an array of the backtrace()
    final public  function getPrevious();       // previous exception
    final public  function getTraceAsString();  // formatted string of trace
    // Overrideable
    public function __toString();               // formatted string for display
}
?>
If a class extends the built-in Exception class and re-defines the constructor, it is highly recommended that it also call parent::__construct() to ensure all available data has been properly assigned. The __toString() method can be overridden to provide a custom output when the object is presented as a string.
Note:
Exceptions cannot be cloned. Attempting to clone an Exception will result in a fatal
E_ERRORerror.
Example #2 Extending the Exception class (PHP 5.3.0+)
<?php
/**
 * Define a custom exception class
 */
class MyException extends Exception
{
    // Redefine the exception so message isn't optional
    public function __construct($message, $code = 0, Exception $previous = null) {
        // some code
    
        // make sure everything is assigned properly
        parent::__construct($message, $code, $previous);
    }
    // custom string representation of object
    public function __toString() {
        return __CLASS__ . ": [{$this->code}]: {$this->message}\n";
    }
    public function customFunction() {
        echo "A custom function for this type of exception\n";
    }
}
/**
 * Create a class to test the exception
 */
class TestException
{
    public $var;
    const THROW_NONE    = 0;
    const THROW_CUSTOM  = 1;
    const THROW_DEFAULT = 2;
    function __construct($avalue = self::THROW_NONE) {
        switch ($avalue) {
            case self::THROW_CUSTOM:
                // throw custom exception
                throw new MyException('1 is an invalid parameter', 5);
                break;
            case self::THROW_DEFAULT:
                // throw default one.
                throw new Exception('2 is not allowed as a parameter', 6);
                break;
            default: 
                // No exception, object will be created.
                $this->var = $avalue;
                break;
        }
    }
}
// Example 1
try {
    $o = new TestException(TestException::THROW_CUSTOM);
} catch (MyException $e) {      // Will be caught
    echo "Caught my exception\n", $e;
    $e->customFunction();
} catch (Exception $e) {        // Skipped
    echo "Caught Default Exception\n", $e;
}
// Continue execution
var_dump($o); // Null
echo "\n\n";
// Example 2
try {
    $o = new TestException(TestException::THROW_DEFAULT);
} catch (MyException $e) {      // Doesn't match this type
    echo "Caught my exception\n", $e;
    $e->customFunction();
} catch (Exception $e) {        // Will be caught
    echo "Caught Default Exception\n", $e;
}
// Continue execution
var_dump($o); // Null
echo "\n\n";
// Example 3
try {
    $o = new TestException(TestException::THROW_CUSTOM);
} catch (Exception $e) {        // Will be caught
    echo "Default Exception caught\n", $e;
}
// Continue execution
var_dump($o); // Null
echo "\n\n";
// Example 4
try {
    $o = new TestException();
} catch (Exception $e) {        // Skipped, no exception
    echo "Default Exception caught\n", $e;
}
// Continue execution
var_dump($o); // TestException
echo "\n\n";
?>
Note:
Versions of PHP 5, prior to PHP 5.3.0 do not support nesting of exceptions. The following code fragment can be used as a replacement MyException class if you wish to run this example.
<?php
/**
* Define a custom exception class
*/
class MyException extends Exception
{
// Redefine the exception so message isn't optional
public function __construct($message, $code = 0) {
// some code
// make sure everything is assigned properly
parent::__construct($message, $code);
}
// custom string representation of object
public function __toString() {
return __CLASS__ . ": [{$this->code}]: {$this->message}\n";
}
public function customFunction() {
echo "A custom function for this type of exception\n";
}
}
?>