Exceptions

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());
}
?>

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();
?>

Just an example why finally blocks are usefull (5.5)

<?php

//without catch
function example() {
  try {
    //do something that throws an exeption
  }
  finally {
    //this code will be executed even when the exception is executed
  }
}

function example2() {
  try {
     //open sql connection check user as example
     if(condition) { 
        return false;
     }
  }
  finally {
    //close the sql connection, this will be executed even if the return is called.
  }
}

?>

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}

If you use the set_error_handler() to throw exceptions of errors, you may encounter issues with __autoload() functionality saying that your class doesn't exist and that's it.

If you do this:

<?php

class MyException extends Exception
{
}

class Tester
{
    public function foobar()
    {
        try
        {
            $this->helloWorld();
        } catch (MyException $e) {
            throw new Exception('Problem in foobar',0,$e);
        }
    }
    
    protected function helloWorld()
    {
        throw new MyException('Problem in helloWorld()');
    }
}

$tester = new Tester;
try
{
    $tester->foobar();
} catch (Exception $e) {
    echo $e->getTraceAsString();
}
?>

The trace will only show $tester->foobar() and not the call made to $tester->helloWorld().

In other words, if you pass a previous exception to a new one, the previous exception's stack trace is taken into account in the new exception.

‘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).

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.

The "finally" block can change the exception that has been throw by the catch block.

<?php
try{
        try {
                throw new \Exception("Hello");
        } catch(\Exception $e) {
                echo $e->getMessage()." catch in\n";
                throw $e;
        } finally {
                echo $e->getMessage()." finally \n";
                throw new \Exception("Bye");
        }
} catch (\Exception $e) {
        echo $e->getMessage()." catch out\n";
}
?>

The output is:

Hello catch in
Hello finally 
Bye catch out

the following is an example of a re-thrown exception and the using of getPrevious function:

<?php

$name = "Name";

//check if the name contains only letters, and does not contain the word name

try
   {
   try
     {
      if (preg_match('/[^a-z]/i', $name)) 
       {
           throw new Exception("$name contains character other than a-z A-Z");
       }   
       if(strpos(strtolower($name), 'name') !== FALSE)
       {
          throw new Exception("$name contains the word name");
       }
       echo "The Name is valid";
     }
   catch(Exception $e)
     {
     throw new Exception("insert name again",0,$e);
     }
   }
 
catch (Exception $e)
   {
   if ($e->getPrevious())
   {
    echo "The Previous Exception is: ".$e->getPrevious()->getMessage()."<br/>";
   }
   echo "The Exception is: ".$e->getMessage()."<br/>";
   }

 ?>

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());
   }
  }

 }
?>

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.

If you are using a namespace, you must indicate the global namespace when using Exceptions.
<?php
namespace alpha;
function foo(){
    throw new \Exception("Something is wrong!");
    // throw new Exception(""); will fail
}

try {
    foo();
} catch( \Exception $e ) {
    // catch( Exception $e ) will give no warning, but will not catch Exception
    echo "ERROR: $e";
}

?>

Just an example why finally blocks are usefull (5.5)

<?php

//without catch
function example() {
  try {
    //do something that throws an exeption
  }
  finally {
    //this code will be executed even when the exception is executed
  }
}

function example2() {
  try {
     //open sql connection check user as example
     if(condition) { 
        return false;
     }
  }
  finally {
    //close the sql connection, this will be executed even if the return is called.
  }
}

Just an example why finally blocks are usefull (5.5)

<?php

//without catch
function example() {
  try {
    //do something that throws an exeption
  }
  finally {
    //this code will be executed even when the exception is executed
  }
}

function example2() {
  try {
     //open sql connection check user as example
     if(condition) { 
        return false;
     }
  }
  finally {
    //close the sql connection, this will be executed even if the return is called.
  }
}

?>

There's some inconsistent behaviour associated with PHP 5.5.3's finally and return statements. If a method returns a variable in a try block (e.g. return $foo;), and finally modifies that variable, the /modified/ value is returned. However, if the try block has a return that has to be evaluated in-line (e.g. return $foo+0;), finally's changes to $foo will /not/ affect the return value.

[code]
function returnVariable(){
    $foo = 1;
    try{
        return $foo;
    } finally {
        $foo++;
    }
}

function returnVariablePlusZero(){
    $foo = 1;
    try{
        return $foo + 0;
    } finally {
        $foo++;
    }
}

$test1 = returnVariable(); // returns 2, not the correct value of 1.
$test2 = returnVariablePlusZero(); // returns correct value of 1, but inconsistent with $test1.
[/code]

It looks like it's trying to be efficient by not allocating additional memory for the return value when it thinks it doesn't have to, but the spec is that finally is run after try is completed execution, and that includes the evaluation of the return expression.

One could argue (weakly) that the first method should be the correct result, but at least the two methods should be consistent.

@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.

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

@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.

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.

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>