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Chapter 11. Exceptions

Table of Contents

Extending 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 11.1. Throwing an Exception

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<?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:

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0.2
Caught exception: Division by zero.
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 11.2. The Built in Exception class

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<?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 11.3. Extending the Exception class

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<?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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