Exceptions

PHP 5 has an exception model similar to that of other programming languages. An exception can be thrown, and caught ("catched") within PHP. Code may be surrounded in a try block, to facilitate the catching of potential exceptions. Each try must have at least one corresponding catch block. Multiple catch blocks can be used to catch different classes of exeptions. Normal execution (when no exception is thrown within the try block, or when a catch matching the thrown exception's class is not present) will continue after that last catch block defined in sequence. Exceptions can be thrown (or re-thrown) within a catch block.

When an exception is thrown, code following the statement will not be executed, and PHP will attempt to find the first matching catch block. If an exception is not caught, a PHP Fatal Error will be issued with an "Uncaught Exception ..." message, unless a handler has been defined with set_exception_handler().

Example #1 Throwing an Exception


<?php

function inverse($x) {

    if (!$x) {

        throw new Exception('Division by zero.');

    }

    else return 1/$x;

}



try {

    echo inverse(5) . "\n";

    echo inverse(0) . "\n";

} catch (Exception $e) {

    echo 'Caught exception: ',  $e->getMessage(), "\n";

}



// Continue execution

echo 'Hello World';

?>

The above example will output:

0.2
Caught exception: Division by zero.
Hello World

Example #2 Nested Exception


<?php



class MyException extends Exception { }



class Test {

    public function testing() {

        try {

            try {

                throw new MyException('foo!');

            } catch (MyException $e) {

                /* rethrow it */

                throw $e;

            }

        } catch (Exception $e) {

            var_dump($e->getMessage());

        }

    }

}



$foo = new Test;

$foo->testing();



?>

The above example will output:

string(4) "foo!"

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 #3 The Built in Exception class


<?php

class Exception

{

    protected $message = 'Unknown exception';   // exception message

    protected $code = 0;                        // user defined exception code

    protected $file;                            // source filename of exception

    protected $line;                            // source line of exception



    function __construct($message = null, $code = 0);



    final function getMessage();                // message of exception 

    final function getCode();                   // code of exception

    final function getFile();                   // source filename

    final function getLine();                   // source line

    final function getTrace();                  // an array of the backtrace()

    final function getTraceAsString();          // formated string of trace



    /* Overrideable */

    function __toString();                       // formated string for display

}

?>

If a class extends the built-in Exception class and re-defines the constructor, it is highly recomended that it also call parent::__construct() to ensure all available data has been properly assigned. The __toString() method can be overriden to provide a custom output when the object is presented as a string.

Example #4 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) {

        // 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";

    }

}





/**

 * 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 isnt 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);

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);

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);

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);

echo "\n\n";

?>