## 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
// 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
/**
* 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;
}
}
}
// 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";
?>`
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