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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 0Exception $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 0Exception $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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