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# Chapter 20. 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 20-1. Throwing an Exception
`<?php try { $error = 'Always throw this error'; throw new Exception($error); // Code following an exception is not executed. echo 'Never executed'; } catch (Exception $e) { echo 'Caught exception: ', $e->getMessage(), "\n"; } // Continue execution echo 'Hello World'; ?>`

# 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 20-2. 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 20-3. 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"; ?>`

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