## Extending Exceptions

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 implements Throwable
    {
        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 = '', $code = 0, ?Throwable $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

        // Overridable
        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_ERROR` error.

Example #2 Extending the Exception class

    <?php
    /**
     * Define a custom exception class
     */
    class MyException extends Exception
    {
        // Redefine the exception so message isn't optional
        public function __construct($message, $code = 0, ?Throwable $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;
            }
        }
    }

    echo "# Example 1\n";
    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\n";
    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\n";
    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\n";
    try {
        $o = new TestException();
    } catch (Exception $e) {        // Skipped, no exception
        echo "Default Exception caught\n", $e;
    }

    // Continue execution
    var_dump($o); // TestException

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