set_exception_handler

(PHP 5, PHP 7)

set_exception_handler — Sets a user-defined exception handler function

Description

callable set_exception_handler ( callable $exception_handler )

Sets the default exception handler if an exception is not caught within a try/catch block. Execution will stop after the exception_handler is called.

Parameters

exception_handler

Name of the function to be called when an uncaught exception occurs. This handler function needs to accept one parameter, which will be the exception object that was thrown. This is the handler signature before PHP 7:

void handler ( Exception $ex )

Since PHP 7, most errors are reported by throwing Error exceptions, which will be caught by the handler as well. Both Error and Exception implements the Throwable interface. This is the handler signature since PHP 7:

void handler ( Throwable $ex )

NULL may be passed instead, to reset this handler to its default state.

Caution

Note that providing an explicit Exception type hint for the ex parameter in your callback will cause issues with the changed exception hierarchy in PHP 7.

Return Values

Returns the name of the previously defined exception handler, or NULL on error. If no previous handler was defined, NULL is also returned.

Changelog

Version	Description
7.0.0	The type of parameter passed into `exception_handler` changed from Exception to Throwable
5.5.0	Previously, if `NULL` was passed then this function returned `TRUE`. It returns the previous handler since PHP 5.5.0.

Examples

Example #1 set_exception_handler() example


<?php
function exception_handler($exception) {
  echo "Uncaught exception: " , $exception->getMessage(), "\n";
}

set_exception_handler('exception_handler');

throw new Exception('Uncaught Exception');
echo "Not Executed\n";
?>

User Contributed Notes

Anonymous

6 years ago


Things you should be aware of:

An exception handler handles exceptions that were not caught before. It is the nature of an exception that it discontinues execution of your program - since it declares an exceptional situation in which the program cannot continue (except you catch it).

Since it has not been catched your code signals it is not being aware of the situation and cant go on.

This implies: returning to the script is simply impossible when the exception handler has already been called, since an uncaught exception is not a notice. use your own debug- or notice-log-system for things like that.

Furthermore: While is is still possible to call functions from your script, since the exception handler has already been called exceptions bubbling from that piece of code won't trigger the exception handler again. php will die without leaving any information apart form "uncaught exception with unknown stack frame". So if you call functions from your script, make sure that you catch any exceptions that possibly occur via try..catch inside the exception handler.

For those of you who misinterpreted the essential meaning of the exception handler: it's only use is to handle the abortion of your script gracefully, e.g. in a project like facebook or wikipedia: render a nice error page, eventually hiding information which shall not leak into the public (instead you may want to write to your log or mail the sys-admin or stuff like that).

In other words: Redirecting all php-errors form an error-handler using exceptions - including notices - is a very dumb idea, if you do not intend having your script aborted everytime you didn't set a variable (for example).

my 2 cents.

Glen

9 years ago


If you want a class instance to handle the exception, this is how you do it :

<?php
class example {
   public function __construct() {
       @set_exception_handler(array($this, 'exception_handler'));
       throw new Exception('DOH!!');
   }

   public function exception_handler($exception) {
       print "Exception Caught: ". $exception->getMessage() ."\n";
   }
}

$example = new example;

?>

See the first post (Sean's) for a static example.  As Sean points out, the exception_handler function must be declared public.

joshua dot boyle-petrie at its dot monash dot edu

7 years ago


Thanks to mastabog we know that throwing an exception within the exception handler will trigger a fatal error and a debugging nightmare.  To avoid throwing an exception within there should be easy.

However, if you use a custom error handler to convert errors to ErrorExceptions suddenly there are a multitude of new ways to accidentally throw exceptions within the exception handler.

<?php
function error_handler($code, $message, $file, $line)
{
    if (0 == error_reporting())
    {
        return;
    }
    throw new ErrorException($message, 0, $code, $file, $line);
}
function exception_handler($e)
{
    // ... normal exception stuff goes here
    print $undefined; // This is the underlying problem
}
set_error_handler("error_handler");
set_exception_handler("exception_handler");
throw new Exception("Just invoking the exception handler");
?>
Output: Fatal error: Exception thrown without a stack frame in Unknown on line 0

The best way I have found to avoid this is to wrap up everything in the exception handler in a try/catch block.
<?php
function exception_handler($e)
{
    try
    {
        // ... normal exception stuff goes here
        print $undefined; // This is the underlying problem
    }
    catch (Exception $e)
    {
        print get_class($e)." thrown within the exception handler. Message: ".$e->getMessage()." on line ".$e->getLine();
    }
}
?>
Output: ErrorException thrown within the exception handler. Message: Undefined variable: undefined on line 14

This speeds up debugging and offers some scalability to any other exceptions accidentally thrown within the exception handler.

Another solution is to restore the error handler at the beginning of the exception handler.  While this is a silver bullet in terms of avoiding the ErrorExceptions, debugging messages then rely on the error_reporting() level and the display_errors directive.  Why mention this?  It might be preferable for production code since we care more about hiding errors from users than convenient debugging messages.

parazuce at gmail dot com

8 years ago


I've been messing around with this function, and have noticed you can pass an anonymous function (created with create_function()) through as the default handler, for example:

set_exception_handler(create_function('$e', 'exit("An unknown error occurred");'));

That snippet of code can be used if you simply want to suppress all exceptions that are not handled.  This can be a great thing, because secure data could possibly be leaked otherwise (for example, the default exception handler could output a snippet of your SQL code that was involved with the exception being thrown).

You will want to use this wisely, however (if at all).

sean at seanodonnell dot com

10 years ago


Using the 'set_exception_handler' function within a class, the defined 'exception_handler' method must be declared as 'public' (preferrable 'public static' if you use the "array('example', 'exception_handler')" syntax). 



<?php

class example {

    public function __construct() {

        @set_exception_handler(array('example', 'exception_handler'));

        throw new Exception('DOH!!');

    }



    public static function exception_handler($exception) {

        print "Exception Caught: ". $exception->getMessage() ."\n";

    }

}



$example = new example;



echo "Not Executed\n";

?> 



Declaring the 'exception_handler' function as 'private' causes a FATAL ERROR.



[derick: red. updated statement about static a bit]

mastabog at hotmail dot com

9 years ago


A behaviour not documented or discussed enough, yet pretty common is that is that if an exception is thrown from the global exception handler then a fatal error occurs (Exception thrown without a stack frame). That is, if you define your own global exception handler by calling set_exception_handler() and you throw an exception from inside it then this fatal error occurs. It is only natural though, as the callback defined by set_exception_handler() is only called on uncaught (unhandled) exceptions so if you throw one from there then you get this fatal error as there is no exception handler left (you override the php internal one by calling set_exception_handler()), hence no stack frame for it.

Example:

<?php

function myExceptionHandler (Exception $ex)
{
    throw $ex;
}

set_exception_handler("myExceptionHandler");

throw new Exception("This should cause a fatal error and this message will be lost");

?>

Will cause a Fatal error: Exception thrown without a stack frame

If you skip/comment the set_exception_handler("...") line then the internal PHP global handler will catch the exception and output the exception message and trace (as string) to the browser, allowing you to at least see the exception message.

While it is a very good idea to always define your own global exception handler by using the set_exception_handler() function, you should pay attention and never throw an exception from it (or if you do then catch it).

Finally, every serious coder should use an IDE with debugging capabilities. Tracking down an error like this becomes a trivial matter by using simple debugging "Step into" commands (I for one recommend Zend IDE v5.2 at the moment of this writing). I have seen numerous messages on the internet with people wondering why this message pops up.

Cheers

p.s. Other causes for this error which are somehow unrelated to this is when you throw an exception from a destructor (the reasons behind that are similar though, the global handler might no longer exist due to the php engine shutting the page down).

Josef Spak

1 year ago


On GNU/Linux, When an exception handler is called, PHP will end with exit status code 0 instead of 255.

You can change the exit status code with an exit() call at the end of your custom error handler.

marques at displague dot com

8 years ago


frank,

Your exception handler is configured to be the handler for all exceptions, yet if a basic 'Exception' is thrown, your static method will error because 'Exception's do not have 'getException'.  Because of this I don't see a real purpose to making the uncaught handler a class that extends Exception.  

I do like the idea of using static methods of a general Exception handling class.  

<?php
class ExceptionHandler {   
    public static function printException(Exception $e)
    {
        print 'Uncaught '.get_class($e).', code: ' . $e->getCode() . "<br />Message: " . htmlentities($e->getMessage())."\n";
    }
   
    public static function handleException(Exception $e)
    {
         self::printException($e);
    }
}

set_exception_handler(array("ExceptionHandler", "handleException"));

class NewException extends Exception {}
try {
  throw new NewException("Catch me once", 1);
} catch (Exception $e) {
  ExceptionHandler::handleException($e);
}

throw new Exception("Catch me twice", 2);
?>

Gives:
Uncaught NewException, code: 1<br />Message: Catch me once
Uncaught Exception, code: 2<br />Message: Catch me twice

There are much more interesting things that can be done like reformating and optionally displaying or emailing them.  But this class acts a nice container for those functions.

pinkgothic at gmail dot com

5 years ago


If you're handling sensitive data and you don't want exceptions logging details such as variable contents when you throw them, you may find yourself frustratedly looking for the bits and pieces that make up a normal stack trace output, so you can retain its legibility but just alter a few things. In that case, this may help you:

<?php

function exceptionHandler($exception) {

    // these are our templates
    $traceline = "#%s %s(%s): %s(%s)";
    $msg = "PHP Fatal error:  Uncaught exception '%s' with message '%s' in %s:%s\nStack trace:\n%s\n  thrown in %s on line %s";

    // alter your trace as you please, here
    $trace = $exception->getTrace();
    foreach ($trace as $key => $stackPoint) {
        // I'm converting arguments to their type
        // (prevents passwords from ever getting logged as anything other than 'string')
        $trace[$key]['args'] = array_map('gettype', $trace[$key]['args']);
    }

    // build your tracelines
    $result = array();
    foreach ($trace as $key => $stackPoint) {
        $result[] = sprintf(
            $traceline,
            $key,
            $stackPoint['file'],
            $stackPoint['line'],
            $stackPoint['function'],
            implode(', ', $stackPoint['args'])
        );
    }
    // trace always ends with {main}
    $result[] = '#' . ++$key . ' {main}';

    // write tracelines into main template
    $msg = sprintf(
        $msg,
        get_class($exception),
        $exception->getMessage(),
        $exception->getFile(),
        $exception->getLine(),
        implode("\n", $result),
        $exception->getFile(),
        $exception->getLine()
    );

    // log or echo as you please
    error_log($msg);
}

?>

If you're not a fan of sprintf() or the duplicate $exception->getFile() and $exception->getLine() calls you can of course replace that as you like - consider this a mere compilation of the parts.

Anonymous

2 years ago


By default the stack trace is pretty unreadable, so you might want to wrap it in <pre> tags. Here I also wrap it in a <div> and set the class 'alert alert-danger' which are CSS classes in the Bootstrap CSS framework to style them red.
<?php

function exception_handler($exception) {
  echo '<div class="alert alert-danger">';
  echo '<b>Fatal error</b>:  Uncaught exception \'' . get_class($exception) . '\' with message ';
  echo $exception->getMessage() . '<br>';
  echo 'Stack trace:<pre>' . $exception->getTraceAsString() . '</pre>';
  echo 'thrown in <b>' . $exception->getFile() . '</b> on line <b>' . $exception->getLine() . '</b><br>';
  echo '</div>';
}

set_exception_handler('exception_handler');

mc-php-doco at oak dot homeunix dot org

10 years ago


This seems not to work when calling the PHP binary with the '-r' flag.

For example, if I run it like this:

    php -r '
      function exception_handler($exception) {
        echo "Uncaught exception: " , $exception->getMessage(), "\n";
      }
 
      set_exception_handler("exception_handler");
 
      throw new Exception("Uncaught Exception");
      echo "Not Executed\n";
    '

Or if I place it in a file and run it like this:

    php -r 'include "./tmp.php";'

I get a stack trace instead of having the function 'exception_handler' called.  If run it like this:

    php tmp.php

It works fine.  

(Why run code from '-r'?  Sometimes it's useful to add stuff around the include like calls to microtime for benchmarks, or to include a library and then call a few functions from the library, all in an ad-hoc way without having to create new files.)

PHP versions 5.1.2 and 5.0.4.

Anonymous

3 years ago


As of PHP 5.4.11....



Instead of:



Output: Fatal error: Exception thrown without a stack frame in Unknown on line 0



This snippet:



<?php

function error_handler($code, $message, $file, $line)

{

    if (0 == error_reporting())

    {

        return;

    }

    throw new ErrorException($message, 0, $code, $file, $line);

}

function exception_handler($e)

{

    // ... normal exception stuff goes here

    print $undefined; // This is the underlying problem

}

set_error_handler("error_handler");

set_exception_handler("exception_handler");

throw new Exception("Just invoking the exception handler");

?>



Now returns:



Fatal error: Uncaught exception 'ErrorException' with message 'Undefined variable: undefined' in C:\Apache2\htdocs\error\test.php:13 Stack trace: #0 C:\Apache2\htdocs\error\test.php(13): error_handler(8, 'Undefined varia...', 'C:\Apache2\htdo...', 13, Array) #1 [internal function]: exception_handler(Object(Exception)) #2 {main} thrown in C:\Apache2\htdocs\error\test.php on line 13



So it appears that exceptions thrown within exception handler now bypass exception handler.

frank at netventures dot com dot au

8 years ago


Hey all, i've just started to use the exception suite instead of the normal PHP error suite. For those of you looking for an object orientated way to do this without looking down at Glen and Sean's examples (Lesson 1: ALWAYS read the logs!), here you go:

<?php

class NewException extends Exception
{
    public function __construct($message, $code=NULL)
    {
        parent::__construct($message, $code);
    }
    
    public function __toString()
    {
        return "Code: " . $this->getCode() . "<br />Message: " . htmlentities($this->getMessage());
    }
    
    public function getException()
    {
        print $this; // This will print the return from the above method __toString()
    }
    
    public static function getStaticException($exception)
    {
         $exception->getException(); // $exception is an instance of this class
    }
}

set_exception_handler(array("NewException", "getStaticException"));
throw new NewException("Catch me!!!", 69);

?>

Let me know if i'm missing something obvious as I left my glasses at home and I just came back from the Melbourne cup (If I won then I wouldn't be at work still!).

ch at westend dot com

10 years ago


It seems that although the Exception contains a backtrace itself, the stack for the debug_backtrace() function is empty when entering an exception handler. This is very inconvinient. See bug #36477.

Anonymous

6 years ago


When an uncaught exception is handled, execution does NOT return to the script, but rather (unexpectedly, on my end anyway) terminates.

I am using set_error_handler() and set_exception_handler() in conjunction for a system I am currently developing (on v5.3.0 with Xampp). Lets say two E_USER_NOTICES are triggered, the script will die after the first one is processed.

<?php
    set_exception_handler( 'exc_handler' );
    function exc_handler($exception) {
        echo "Uncaught exception: " , $exception->getMessage(), "\n";
    }
    function errorone() {
        throw new Exception("Test 1");
    }
    function errortwo() {
        throw new Exception("Test 2");
    }
    function test() {
        errorone();
        errortwo();
    }
    test();
    test();
?>

Instead of printing (as I'd expect) "Uncaught exception: Text 1\nUncaught exception: Text 2\nUncaught exception: Text 1\nUncaught exception: Text 2\n"

It is only printed ONCE. I've tried a number of different things, but I can't figure out how to return execution to the script after the EXCEPTION HANDLER has run.

If anyone has a solution on how to return execution (hence, allow the script to log uncaught exceptions but continue processing) then PLEASE email me! Thanks!

reg dot php dot manual at entropy dot ch

8 years ago


In my experience, the static keyword is crucial for error handlers which are methods of a class instead of free-standing functions.

    static function exceptionHandler($exception)

works but 

    function exceptionHandler($exception)

doesn't and results in a "Fatal error: Exception thrown without a stack frame in Unknown on line 0" message.

"public" is optional as it is the default anyway (but it is probably clearer to write it explicitly).

set_exception_handler

Description

Parameters

Return Values

Changelog

Examples

See Also

User Contributed Notes