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
{
    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_ERROR error.

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"; } } ?>

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https://secure.php.net/manual/en/language.exceptions.extending.php