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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

<?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


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

<?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

<?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";
?>