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Extending Exceptions> <FAQ: things you need to know about namespaces
[edit] Last updated: Fri, 22 Jul 2011

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Exceptions

Table of Contents

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().

The thrown object must be an instance of the Exception class or a subclass of Exception. Trying to throw an object that is not will result in a PHP Fatal Error.

Note:

Internal PHP functions mainly use Error reporting, only modern Object oriented extensions use exceptions. However, errors can be simply translated to exceptions with ErrorException.

Tip

The Standard PHP Library (SPL) provides a good number of built-in exceptions.

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!"


add a note add a note User Contributed Notes Exceptions
contact at xpertmailer dot com 07-Jul-2011 01:19
To continue the execution code after throw new Exception, goto operator can be used, like this:
<?php
try {
    echo
'one';
    throw new
Exception('-error-'); a:
    echo
'two';
} catch (
Exception $e) {
    echo
$e->getMessage();
    goto
a;
}
//output: one-error-two
?>
But, goto operator can NOT be evaluate or get a value:
<?php
eval('goto a;'); //Fatal error: 'goto' to undefined label 'a'
$b = 'a';
goto
$b; //Parse error: syntax error, unexpected T_VARIABLE, expecting T_STRING
?>
Johan 05-May-2011 08:18
Custom error handling on entire pages can avoid half rendered pages for the users:

<?php
ob_start
();
try {
   
/*contains all page logic
    and throws error if needed*/
   
...
} catch (
Exception $e) {
 
ob_end_clean();
 
displayErrorPage($e->getMessage());
}
?>
fabian dot pijcke at gmail dot com 08-Sep-2010 09:31
Be careful when you set variables inside a try block, they are still declared outside the block.

Note that it is the case for every block constructions in PHP, but it can be useful to be aware of it :)

<?php

try {
       
$a = "blabla";
} catch (
Exception $e) {}

echo isset(
$a) ? ":)" : ":("; // Gives ":)"
echo isset($b) ? ":)" : ":("; // Gives ":("

?>
mcnaney at gmail 31-Aug-2010 05:21
Watch your namespace if you have created your own exception class.

For example:

Error.php
<?php
namespace Global;
class
Error extends Exception {}
?>

Foo.php
<?php
namespace Foo;

try {
 
$this->foo();
} catch (
Error $e) {
   echo
$e->getMessage();
}
?>

You WILL NOT get an error message if Error is not in the Foo namespace. Instead the Exception will be passed upwards.

Should be:
<?php
try {
 
$this->foo();
} catch (Global\
Error $e) {
   echo
$e->getMessage();
}
?>
zmunoz at gmail dot com 18-May-2010 08:05
When catching an exception inside a namespace it is important that you escape to the global space:

<?php
 
namespace SomeNamespace;

 class
SomeClass {

  function
SomeFunction() {
   try {
    throw new
Exception('Some Error Message');
   } catch (\
Exception $e) {
   
var_dump($e->getMessage());
   }
  }

 }
?>
mike at clove dot com 10-Apr-2010 03:38
The PHP documentation has gone from very useful to hideously obstructive.

The people who are rearranging the doc into little, tiny chunks which are hyperlinked all over the place obviously never write code.

I just spent 10 minutes trying to find the name of an IO Exception so I can use it in some code I'm writing.

Old Doc: I would go to the index, click on Exceptions and then scroll down the page (or do a find on IO) and there it would be. 10 seconds tops.

New Doc: Go to the index click on Predefined Exceptions
 Click on Exception - find description of Exception Object - info not there
 Back Button
 Click on Error Exception - find description of Generic ErrorExeption object
 Back Button
 Click on SPL Exceptions (what the hell is this? - something new?)
 Look at Table of contents: 13 Exception Categories - none of which
  looks like an IOException
 Click on Predefined Exceptions in the See Also -
   Back to Previous Useless Page

First You completely screw up the Perl Regular Expression page by chopping it into tiny, obscure chunks and now you destroy the exception documentation.

PLEASE put it back the way it was.

Or get somebody who actually uses this stuff like a handbook while writing code to fix it

Or shoot somebody.

Incredibly frustrated and thinking of rewriting everything in Python,
Mike Howard <mike at clove dot com>
test30zxc at gmail dot com 03-Jan-2010 12:56
If you are using exceptions, code which will store uncaught exceptions might be nice for you,
for example,
PDO throws an exception, but you thought that this is impossible, so you were not even thinking about using try{...}catch(...){...},

<?php
function exception_handler($exception) {
ob_start();
print_r($GLOBALS);
print_r($exception);
 
file_put_contents('exceptions.txt', ob_get_clean(). "\n",FILE_APPEND);
}
set_exception_handler('exception_handler');
?>

to save a uncatched exception simply write
<?php throw new Exception('error'); ?>
somewhere in code.
If we have a lot of exceptions, file will get a huge size in few minutes,
but saving this kind of information may save you from, for example, being sql-injected (attempt to try to inject-sql will be saved)

PS PDO ofc. must be set to throw exceptions.
ask at nilpo dot com 27-May-2009 07:19
If you intend on creating a lot of custom exceptions, you may find this code useful.  I've created an interface and an abstract exception class that ensures that all parts of the built-in Exception class are preserved in child classes.  It also properly pushes all information back to the parent constructor ensuring that nothing is lost.  This allows you to quickly create new exceptions on the fly.  It also overrides the default __toString method with a more thorough one.

<?php
interface IException
{
   
/* Protected methods inherited from Exception class */
   
public function getMessage();                 // Exception message
   
public function getCode();                    // User-defined Exception code
   
public function getFile();                    // Source filename
   
public function getLine();                    // Source line
   
public function getTrace();                   // An array of the backtrace()
   
public function getTraceAsString();           // Formated string of trace
   
    /* Overrideable methods inherited from Exception class */
   
public function __toString();                 // formated string for display
   
public function __construct($message = null, $code = 0);
}

abstract class
CustomException extends Exception implements IException
{
    protected
$message = 'Unknown exception';     // Exception message
   
private   $string;                            // Unknown
   
protected $code    = 0;                       // User-defined exception code
   
protected $file;                              // Source filename of exception
   
protected $line;                              // Source line of exception
   
private   $trace;                             // Unknown

   
public function __construct($message = null, $code = 0)
    {
        if (!
$message) {
            throw new
$this('Unknown '. get_class($this));
        }
       
parent::__construct($message, $code);
    }
   
    public function
__toString()
    {
        return
get_class($this) . " '{$this->message}' in {$this->file}({$this->line})\n"
                               
. "{$this->getTraceAsString()}";
    }
}
?>

Now you can create new exceptions in one line:

<?php
class TestException extends CustomException {}
?>

Here's a test that shows that all information is properly preserved throughout the backtrace.

<?php
function exceptionTest()
{
    try {
        throw new
TestException();
    }
    catch (
TestException $e) {
        echo
"Caught TestException ('{$e->getMessage()}')\n{$e}\n";
    }
    catch (
Exception $e) {
        echo
"Caught Exception ('{$e->getMessage()}')\n{$e}\n";
    }
}

echo
'<pre>' . exceptionTest() . '</pre>';
?>

Here's a sample output:

Caught TestException ('Unknown TestException')
TestException 'Unknown TestException' in C:\xampp\htdocs\CustomException\CustomException.php(31)
#0 C:\xampp\htdocs\CustomException\ExceptionTest.php(19): CustomException->__construct()
#1 C:\xampp\htdocs\CustomException\ExceptionTest.php(43): exceptionTest()
#2 {main}
rctay89 at gmail dot com 17-Dec-2008 09:33
The internal implementation of Exception.__toString is something like this:

<?php

class MyException extends Exception {
  public function
__toString() {
    return
"exception '".__CLASS__ ."' with message '".$this->getMessage()."' in ".$this->getFile().":".$this->getLine()."\nStack trace:\n".$this->getTraceAsString();
  }
}

?>

Useful if you want to customize your exception toString format, but not to deviate too much from the built-in one.
Shot (Piotr Szotkowski) 21-Oct-2008 07:13
‘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.’

‘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().’

These two sentences seem a bit contradicting about what happens ‘when a catch matching the thrown exception’s class is not present’ (and the second sentence is actually correct).
i dot crash17 at gmail dot com 05-Apr-2008 04:50
As a response to Anonymous on 28-Dec-2007 07:25, please, remember that creating lots of MyXExtension classes is wrong if and only if your object oriented model is not well designed.

One should (I even think MUST) create as many custom Exception extentions as the design needs, because every different type of exception requires (or will require, even if we haven't think about it) a different response, from showing different messages to the user, to log error messages or even kill the script.

Forwarding can become confusing if you are using a big number of classes and packages, and is useless if you are using few (unless you are too lasy to write 3, 5 or 10 includes and their respective Exception classes).

Remember that object orientation was created to give more semantics to the programming code so the programmer understands better what is written without having to excecute the code, so don't do things that go against that, which is the cool thing about object orientation.
michael dot ochs at gmx dot net 21-Mar-2008 11:44
Actually it isn't possible to do:
<?php
someFunction
() OR throw new Exception();
?>

This leads to a T_THROW Syntax Error. If you want to use this kind of exceptions, you can do the following:

<?php
function throwException($message = null,$code = null) {
    throw new
Exception($message,$code);
}

someFunction() OR throwException();
?>
chugadie dot geo at yahoo dot com 05-Mar-2008 06:40
@webmaster at asylum-et dot com

What Mo is describing is bug 44053 (http://bugs.php.net/bug.php?id=44053) in which exceptions cannot be caught if you are using a custom error handler to catch warnings, notices, etc.
omnibus at omnibus dot edu dot pl 04-Dec-2007 11:11
Just to be more precise in what Frank found:
Catch the exceptions always in order from the bottom to the top of the Exception and subclasses class hierarchy. If you have class MyException extending Exception and class My2Exception extending MyException always catch My2Exception before MyException.

Hope this helps
frank at netventures dot com dot au 07-Nov-2007 11:43
If you are going to use multiple catches within a try-catch then do not forget the stacking order of those catches!

This is important as any classes that extend the Exception class, like MyException in example 20.3, will be caught in the Exception case. This is because your newly extended class also has a class type of Exception. This baffled me for awhile as the examples here worked but mine didn't because my first catch was trying to catch Exception.

Example:

<?php

/**
 * My1Exception extends Exception
 * My2Exception extends Exception
 */

/**
 * This will always fall in the first exception
 */
try {
    throw new
My1Exception("My fail english? That's unpossible", 69);
} catch (
Exception $e) {
    print
"Incorrect Exception";
} catch (
My1Exception $e) {
    print
"Correct Exception but I won't be here";
} catch (
My2Exception $e) {
    print
"Again, incorrect";
}

/**
 * Whereas here, the catch stacking order was changed so our throw will cascade into the correct catch
 */
try {
    throw new
My1Exception("My cat's breath smells like cat food", 69);
} catch (
My2Exception $e) {
    print
"Incorrect Exception";
} catch (
My1Exception $e) {
    print
"Correct Exception and I WILL be printed";
} catch (
Exception $e) {
    print
"Again, incorrect";
}

?>

So, ALWAYS keep the Exception catch block at the bottom, then any of the other extended exceptions that extend from Exception, then any of your other extended exceptions that extend from those extended exceptions, etc
hartym dot dont dot like dot spam at gmail dot com 18-Oct-2007 11:41
@serenity: of course you need to throw exception within the try block, catch will not watch fatal errors, nor less important errors but only exceptions that are instanceof the exception type you're giving. Of course by within the try block, i mean within every functions call happening in try block.

For example, to nicely handle old mysql errors, you can do something like this:

<?php
try
{
 
$connection = mysql_connect(...);
  if (
$connection === false)
  {
    throw new
Exception('Cannot connect do mysql');
  }

  
/* ... do whatever you need with database, that may mail and throw exceptions too ... */

  
mysql_close($connection);
}
catch (
Exception $e)
{
  
/* ... add logging stuff there if you need ... */

 
echo "This page cannot be displayed";
}

?>

By doing so, you're aiming at the don't repeat yourself (D.R.Y) concept, by managing error handling at only one place for the whole.
peter dot goodman at gmail dot com 15-Jun-2007 02:52
I've found that exception destructors are not called unless the exception is caught.

I've created a simple solution to this problem (calling __destruct() from __toString() ) and have written up a lengthy article detailing one good use case for this method at http://ioreader.com/2007/06/14/taking-advantage-of-exceptions-in-php5/

Also, one of the useful things about using a destructor as a clean up method is that it is called at the end of a catch statement.
jon at hackcraft dot net 24-Jan-2007 05:52
Further to dexen at google dot me dot up with "use destructors to perform a cleanup in case of exception". The fact that PHP5 has destructors, exception handling, and predictable garbage collection (if there's a single reference in scope and the scope is left then the destructor is called immediately) allows for the use of the RAII idiom.

http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization and my own http://www.hackcraft.net/RAII/ describe this.
dexen at google dot me dot up 18-Sep-2006 01:45
Summary:
 * use destructors to perform a cleanup in case of exception.

PHP calls method __destruct()  on instance of class when variable storing the instance goes out-of-scope (or gets unset). This works for function leave by Exception, aside of plain return. (same as for C++, AFAIK)

aFunction() {
$i = new LockerClass();
throw new MinorErrorEx('Warn user & perform some other activity');
// $i->__destruct() gets called before stack unwind begins, unlocking whatever get locked by new LockerClass();
 return $bar;
}

(A lengthy) example:

Let's say you need to perform a series of operaions on SQL database that should not get disrupted. You lock the tables:
<?php
function updateStuff() {
   
DB::query('LOCK TABLES `a`, `b`, `c` WRITE');
   
/* some SQL Operations */
   
someFunction();
   
/* more SQL Operations */
   
DB::query('UNLOCK TABLES');
}
?>

Now, let's supouse that someFunction() may throw an exception. This would leave us with the tables locked, as the second DB::query() will not get called. This pretty much will cause the next query to fail. You can do it like:
<?php
function updateStuff() {
   
DB::query('LOCK TABLES `a`, `b` WRITE');
   
/* some SQL Operations */
   
try {
       
someFunction(); }
    catch (
Exception $e ) {
       
DB::query('UNLOCK TABLES');
        throw
$e;
    }
   
/* more SQL Operations */
   
DB::query('UNLOCK TABLES')
}
?>

However, this is rather ugly as we get code duplication. And what if somebody later modifies updateStuff() function in a way it needs another step of cleanup, but forget to add it to catch () {}? Or when we have multiple things to be cleaned up, of which not all will be valid all the time?
 
My solution using destructor: i create an instance of class DB holding a query unlocking tables which will be executed on destruction.

<?php
function updateStuff() {
   
$SQLLocker = DB::locker( /*read lock list*/array('a', 'b'), /*write lock list*/array('b') );
   
/* some SQL Operations */
   
someFunction();
   
/* $SQLLocker gets destructed there if someFunction() throws an exception */
   
DB::query('UNLOCK TABLES');
   
/* other SQL Operations */
    /* $SQLLocker gets destructed there if someFunction() does not throw an exception */
}

class
DB {
    function
locker ( $read, $write ) {
       
DB::query( /*locks*/);
       
$ret = new DB;
       
$ret->onDestruct = 'UNLOCK TABLES';
        return
$ret;
    }

    function
_destructor() {
        if (
$this->onDestruct )
           
DB::query($this->onDestruct);
    }
}
?>
jazfresh at hotmail.com 08-Aug-2006 05:18
Sometimes you want a single catch() to catch multiple types of Exception. In a language like Python, you can specify multiple types in a catch(), but in PHP you can only specify one. This can be annoying when you want handle many different Exceptions with the same catch() block.

However, you can replicate the functionality somewhat, because catch(<classname> $var) will match the given <classname> *or any of it's sub-classes*.

For example:

<?php
class DisplayException extends Exception {};
class
FileException extends Exception {};
class
AccessControl extends FileException {}; // Sub-class of FileException
class IOError extends FileException {}; // Sub-class of FileException

try {
  if(!
is_readable($somefile))
     throw new
IOError("File is not readable!");
  if(!
user_has_access_to_file($someuser, $somefile))
     throw new
AccessControl("Permission denied!");
  if(!
display_file($somefile))
     throw new
DisplayException("Couldn't display file!");

} catch (
FileException $e) {
 
// This block will catch FileException, AccessControl or IOError exceptions, but not Exceptions or DisplayExceptions.
 
echo "File error: ".$e->getMessage();
  exit(
1);
}
?>

Corollary: If you want to catch *any* exception, no matter what the type, just use "catch(Exception $var)", because all exceptions are sub-classes of the built-in Exception.
fjoggen at gmail dot com 26-Apr-2006 08:58
This code will turn php errors into exceptions:

<?php
function exceptions_error_handler($severity, $message, $filename, $lineno) {
    throw new
ErrorException($message, 0, $severity, $filename, $lineno);
}

set_error_handler('exceptions_error_handler');
?>

However since <?php set_error_handler()?> doesn't work with fatal errors, you will not be able to throw them as Exceptions.
jd at wuputah dot com 07-May-2005 02:15
PHP5 supports exception throwing inside a function, and catching it outside that function call. There is no mention of this in documentation but it works just fine, as tested by this sample code:

<?php

function exceptionFunction() {
        throw new
Exception("Throwing an exception!");
}

try {
       
exceptionFunction();
} catch (
Exception $e) {
        echo
"Exception caught!\n";
}

?>

The result in PHP 5.0.3 is "Exception caught!"

Further tests show that nested functions with exceptions, methods throwing exceptions, etc all work the same way. This is like declaring all classes (or methods) in Java as "class ClassName throws Exception". While I consider this a good thing, you should be aware that any thrown exception will propagate up your stack until it is either caught or runs out of stack.

 
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