Copyright © 2008, Zend Technologies Inc.

 

ELEGANT WAYS OF HANDLING PHP ERRORS AND EXCEPTIONSBy Eddo Rotman

Sep 17, 2008 | 2 |

Basically errors can be of one of two types• External Errors• Logic Errors (a.k.a. Bugs)

What about these error types?• External Errors will always occur at some point or another• External Errors which are not accounted for are Logic Errors• Logic Errors are harder to track down

Errors

Sep 17, 2008 | 3 |

PHP Errors

Four levels of error severity to start with• Strict standard problems (E_STRICT)• Notices (E_NOTICE)• Warnings (E_WARNING)• Errors (E_ERROR)

Sep 17, 2008 | 4 |

PHP Errors (cont)

// E_NOTICE$x = $y + 3;// E_WARNING$fh = fopen('thisisnotarealfile', 'r');// E_ERRORnonExistingFunction();

Notice: Undefined variable: y in /home/eddo/workspaces/neon/ZendCon08­ServerIndie/xxx.php on line 6

Warning: fopen(thisisnotarealfile) [function.fopen]: failed to open stream: No such file or directory in /home/eddo/workspaces/neon/ZendCon08­ServerIndie/xxx.php on line 8

Fatal error: Call to undefined function nonexistingfunction() in /home/eddo/workspaces/neon/ZendCon08­ServerIndie/xxx.php on line 10

Sep 17, 2008 | 5 |

User Triggered Errors

Almost the same as the ones in the previous slides• User triggered notice (E_USER_NOTICE)• User triggered warning (E_USER_WARNING)• User triggered error (E_USER_ERROR)

Triggering them is done using trigger_error()For example:

function getFooPlusOne($foo) {if (3 > $foo) {

trigger_error('foo has to be greater than 3', E_USER_ERROR);}return ($foo + 1);

}

Sep 17, 2008 | 6 |

Additional Error TypesCatchable fatal error

• E_RECOVERABLE_ERROR – a probably dangerous error occurred. If not handled by the user, the application will abort as if this was an E_ERROR

Parsing errors

• E_PARSE – there is a syntactic error found while parsing the script. This is a fatal error

Compilation errors

• E_COMPILE_ERROR – a fatal error occurred in the engine while compiling the script

• E_COMPILE_WARNING - a nonfatal error occurred in the engine while compiling the script

PHP core errors

• E_CORE_ERROR – a fatal runtime error occurred in the engine

• E_CORE_WARNING – a nonfatal runtime error occurred in the engine

Sep 17, 2008 | 7 |

Error Reporting Settings

Setting which errors PHP will report is done through the error_reporting directive

• in php.ini fileerror_reporting = E_ALL & ~E_NOTICE

• in runtimeerror_reporting(E_ALL & ~E_NOTICE);

• in .htaccess or apache.confphp_value error_reporting 6135

Sep 17, 2008 | 8 |

Handling the Errors

There are four ways to handle errors

• Display them• Log them• Ignore them• Act on them

Sep 17, 2008 | 9 |

Displaying Errors

How to display errors in the standard output -

• Set the display_errors directive to On

• Set the error_reporting to the appropriate severity level

Displaying errors is good for the programmer, bad for the user

Sep 17, 2008 | 10 |

Displaying Errors (cont)

Sep 17, 2008 | 11 |

Displaying Errors (cont)

Sep 17, 2008 | 12 |

Logging Errors

How to set PHP to automatically log errors

• Set the log_errors directive to On

• Set the error_log directive to your preferred logging option

PHP supports two options for logging errors

• Logging to a file – set the error_log to a file path

• Logging to syslog – set the error_log to syslog

Sep 17, 2008 | 13 |

Ignoring Errors

Don't do that.

Sep 17, 2008 | 14 |

Acting on Errors

PHP enables us to set a default error handler using the set_error_handler() function

Five parameters will be passed to the user-defined error handler function

• integer $errno – error severity level• string $errstr – error message• string $errfile [optional] – filename where the error was raised• integer $errline [optional] – line number where the error was

raised• array $errcontext [optional] - an array of every variable that

existed in the scope the error was triggered in

Sep 17, 2008 | 15 |

Acting on Errors (cont)

function demoErrorHandler($errno, $errstr, $errfile, $errline) {switch ($errno) {

case E_USER_ERROR:Logger::log(E_ERROR, $errstr, $errfile, $errline);require_once(FULL_PATH_DEFAULT_ERROR_PAGE);exit(1); // control the flowbreak;

case E_WARNING:case E_USER_WARNING:

Logger::log(E_WARNING, $errstr, $errfile, $errline);break;

case E_NOTICE:case E_USER_NOTICE:

Logger::log(E_NOTICE, $errstr, $errfile, $errline);break;

default:Logger::log(0, $errstr, $errfile, $errline);break;

}return true; // Avoid running PHP's internal error handler

}

set_error_handler("demoErrorHandler");

Sep 17, 2008 | 16 |

Acting on Errors (cont)

What can the error handler do?

• Display a safer message to the user

• Insert the data into a DB

• Write to a file

• Send an email

• ...

Keep in mind that on nonfatal errors, your script will keep on running

Sep 17, 2008 | 17 |

Handling External Errors

External errors will always occur at some point of an application's life-cycle

External errors which are not accounted for are bugsfor example:

• Assuming a DB connection always succeeds

• Assuming a file is opened properly

• Assuming an XML file has the right format

• ...

Assumption is the big mama of all....

Sep 17, 2008 | 18 |

Handling External Errors (cont)

$fh = fopen($myfile, 'w');if ($fh) {

$fh­>write('save the rhinos!');} else {

redirectToErrorPage('Failed opening an important file');die(1);

}

$fh = @fopen($myfile, 'w');$fh ->fwrite('save the rhinos!');

$db = mysql_connect();mysql_query('SELECT * FROM users WHERE id=18');

$db = mysql_connect();if (! $db) {

redirectToErrorPage('Could not connect to the database!');die(1);

}mysql_query('SELECT * FROM users WHERE id=18', $db);

Sep 17, 2008 | 19 |

Zend Monitor

Sep 17, 2008 | 20 |

Exceptions

An Exception can be thought of as a flow-control structure, or as an error control mechanism

• Exceptions should be used to handle logic errors• Exceptions may be considered as any other type of flow-

control syntax (such as if-else, while and foreach)• Exceptions are slower and consume more memory than other

flow-control syntaxes, therefore it is not recommended to use it as a flow-control structure per se

Unhandled Exceptions are fatal errors

Sep 17, 2008 | 21 |

Exceptions (cont)

Sep 17, 2008 | 22 |

Exceptions (cont)

Exceptions are classes and therefore you may extend them to fit your needs or serve as markers

class DataBaseException extends Exception {

}

class MathException extends Exception {

}

Sep 17, 2008 | 23 |

Handling Exceptions

Terminology:• throw – the act of publishing an Exception• try block – a segment of the code which may have an

exception thrown in it• catch block – a segment of the code which handles an

exception if one happens• finally – is not available in PHP, but common in other

languages

Sep 17, 2008 | 24 |

Handling Exceptions (cont)

try {

if (0 == $denominator) {throw new Exception('Zero denominator');

}

echo ($numerator / $denominator); 

} catch (Exception $e) {

echo 'You can not divide by zero';die; // make sure the script stops

}

Sep 17, 2008 | 25 |

Handling Errors (cont)

class Calculator {

/** * @param float $numerator * @param float $denominator * @return float * @throws MathException */function divide($numerator, $denominator) {

if (0 == $denominator) {throw new MathException('Zero denominator');

}

return ($numerator / $denominator);}

}

Sep 17, 2008 | 26 |

Handling Exceptions (cont)

try {$calc = new Calculator();echo $calc­>divide($numerator, $denominator);

} catch (MathException $e) {echo 'A mathematic integrity failure: ', $e­>getMessage();

} catch (Exception $e) {echo 'A system error: ', $e­>getMessage()

}echo 'Done';

It is possible to have several catch block for one try block where each is intended to catch a different type of Exception

Sep 17, 2008 | 27 |

Exception Hierarchies

Since Exceptions are objects, i.e. instances of classes, you should take advantage of class hierarchy capabilities

e.g. have Db2Exception, MysqlException etc. extend DataBaseException

try {$user = new User($username);$user­>authenticate($password);$user­>getAccountBalance();

} catch (UserAuthenticationException $e) {echo "The user is not logged in";

} catch (DataBaseException $e) {Logger::logException('DB Error', $e);echo "The system has encounter some internal error";

} catch (Exception $e) {Logger::logException('Unknown error', $e);echo "The system has encounter some internal error";

}

Sep 17, 2008 | 28 |

Advanced Exceptions

The basic Exception class is a written in C and most of its methods are defined as final

Since it is a PHP class it may be extended to fit your needs. You may add functions and attributes to it

You may only override its __toString() method

Sep 17, 2008 | 29 |

Advanced Exceptions (cont)class MysqlException extends Exception {

private $comment = 'Zend Conference 2008 Example';private $backtrace;

public function __construct() {$this­>code = mysql_errno(); $this­>message = mysql_error();$this­>backtrace = debug_backtrace();

}

public function __toString() {return 'Papa was a Rolling Stone';

}}

try {if (! mysql_connect()) {

throw new MysqlException();}

} catch (MysqlException $e) {echo $e->getMessage();

} catch (Exception $e) {// do something else

}

Sep 17, 2008 | 30 |

Cascading Exceptions

Exceptions bubble up until they are caught in the first catch block which wraps them. As mentioned before, uncaught Exceptions are fatal errors

Use this behavior to cascade Exceptions, i.e. catch them in a smaller logic frame and bubble only the needed data up

Sep 17, 2008 | 31 |

Cascading Exceptions (cont)

class User {public staticfunction fetch($username, $password) {

try {$sql = "SELECT username FROM users WHERE “;$sql.= “ (username={$username} AND password={$password}) “;$sql.= “ LIMIT 1";return (MysqlAdapter::fetch($sql));

} catch (DataBaseException $e) {Logger::logException('Db Error', $e);throw new UserExeption('Unable to authenticate the user');

}return false;

}}

try {$user = User::fetch('Eddo', 'karnaf');

} catch (Exception $e) {// redirect to an error pagedie;

}

Sep 17, 2008 | 32 |

Constructors

PHP will always return an instance of the class when the constructor is called

Throwing an exception in a constructor will enable us to distinguish between a successful construction and a failure

If an Exception is called in a constructor, the destructor is not called

Sep 17, 2008 | 33 |

Constructors (cont)

class User {

private $name;private $data;

public function __construct($name) {$this­>name = (string)$name;$this­>data  = UserModel::getDataByName($name);

if (empty($this­>data)) {throw new Exception("The system failed for {$name}");

}}

}

try  {$user = new User('Eddo Rotman');

} catch (Exception $e) {throw new Exception('Could not find the user');

}

Sep 17, 2008 | 34 |

Default Exception Handler

This is a user-defined top-level Exception Handler which will handle any uncaught Exception

Unlike a try-catch block, after handling an Exception with the default error handler, the script halts

Keep in mind that the default exception handler can not catch all uncaught exceptions which may happen in your code

Sep 17, 2008 | 35 |

Default Exception Handler

function  myDefaultExceptionHandler($exception) {// do something about it

}

set_exception_handler('myDefaultExceptionHandler');

class  MyExcpetionHandling {

public static function doSomething($exception) {// do something about it

}}

set_exception_handler(array('MyExcpetionHandling', 'doSomething'));

Sep 17, 2008 | 36 |

Conclusions

• Errors happen, but it doesn't mean they should be ignored

• Watch out for external errors or they may turn to bugs

• Use Exceptions to better handle errors and logic flaws

• Use Exceptions to distinguish between different error cases

• Have a default error handler and a default exception handler, even if you are sure that everything is covered

Sep 17, 2008 | 37 |

Questions?