PHP 5.5.17 is available

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.

Nota:

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";
?>

Nota:

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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User Contributed Notes 6 notes

up
4
iamhiddensomewhere at gmail dot com
4 years ago
As previously noted exception linking was recently added (and what a god-send it is, it certainly makes layer abstraction (and, by association, exception tracking) easier).

Since <5.3 was lacking this useful feature I took some initiative and creating a custom exception class that all of my exceptions inherit from:

<?php

class SystemException extends Exception
{
    private
$previous;
   
    public function
__construct($message, $code = 0, Exception $previous = null)
    {
       
parent::__construct($message, $code);
       
        if (!
is_null($previous))
        {
           
$this -> previous = $previous;
        }
    }
   
    public function
getPrevious()
    {
        return
$this -> previous;
    }
}

?>

Hope you find it useful.
up
1
sapphirepaw.org
4 years ago
Support for exception linking was added in PHP 5.3.0. The getPrevious() method and the $previous argument to the constructor are not available on any built-in exceptions in older versions of PHP.
up
1
michaelrfairhurst at gmail dot com
1 year ago
Custom exception classes can allow you to write tests that prove your exceptions
are meaningful. Usually testing exceptions, you either assert the message equals
something in which case you can't change the message format without refactoring,
or not make any assertions at all in which case you can get misleading messages
later down the line. Especially if your $e->getMessage is something complicated
like a var_dump'ed context array.

The solution is to abstract the error information from the Exception class into
properties that can be tested everywhere except the one test for your formatting.

<?php

class TestableException extends Exception {

        private
$property;

        function
__construct($property) {

               
$this->property = $property;
               
parent::__construct($this->format($property));

        }

        function
format($property) {
                return
"I have formatted: " . $property . "!!";
        }

        function
getProperty() {
                return
$this->property;
        }

}

function
testSomethingThrowsTestableException() {
        try {
                throw new
TestableException('Property');
        } Catch (
TestableException $e) {
               
$this->assertEquals('Property', $e->getProperty());
        }
}

function
testExceptionFormattingOnlyOnce() {
       
$e = new TestableException;
       
$this->assertEquals('I have formatted: properly for the only required test!!',
               
$e->format('properly for the only required test')
        );
}

?>
up
0
Dor
2 years ago
It's important to note that subclasses of the Exception class will be caught by the default Exception handler

<?php
   
   
/**
     * NewException
     * Extends the Exception class so that the $message parameter is now mendatory.
     *
     */
   
class NewException extends Exception {
       
//$message is now not optional, just for the extension.
       
public function __construct($message, $code = 0, Exception $previous = null) {
           
parent::__construct($message, $code, $previous);
        }
    }
   
   
/**
     * TestException
     * Tests and throws Exceptions.
     */
   
class TestException {
        const
NONE = 0;
        const
NORMAL = 1;
        const
CUSTOM = 2;
        public function
__construct($type = self::NONE) {
            switch (
$type) {
                case
1:
                    throw new
Exception('Normal Exception');
                    break;
                case
2:
                    throw new
NewException('Custom Exception');
                    break;
                default:
                    return
0; //No exception is thrown.
           
}
        }
    }
   
    try {
       
$t = new TestException(TestException::CUSTOM);
    }
    catch (
Exception $e) {
       
print_r($e); //Exception Caught
   
}
   
?>

Note that if an Exception is caught once, it won't be caught again (even for a more specific handler).
up
-1
hollodotme
3 months ago
Not mentioned in the class structure on top of this site are the following members:

<?php
/**
* @var string
*/
protected $string;

/**
* @var array
*/
protected $trace;

/**
* @var Exception
*/
protected $previous;
?>

... and these methods:

<?php
/**
* @return Exception|null
*/
final public function getPrevious();
?>
up
-7
paragdiwan at gmail dot com
5 years ago
I have written similar simple custom exception class. Helpful for newbie.
<?php
   
/*
        This is written for overriding the exceptions.
        custom exception class
    */
   
error_reporting(E_ALL-E_NOTICE);
    class
myCustomException extends Exception
   
{
       
        public function
__construct($message, $code=0)
        {
           
parent::__construct($message,$code);
        }   

        public function
__toString()
        {
            return
"<b style='color:red'>".$this->message."</b>";
        }
       
       
    }

    class
testException
   
{
       
        public function
__construct($x)
        {
       
           
$this->x=$x;
           
        }
       
        function
see()
        {
           

            if(
$this->x==9 )
            {
                throw new
myCustomException("i didnt like it");
            }
        }
    }

   
$obj = new testException(9);
    try{
   
       
$obj->see();
    }
    catch(
myCustomException $e)
    {
        echo
$e;
    }
?>
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