1. Martin Kruliš Who is General Failure and why is he reading my disk? 19. 3. 2015 by Martin Kruliš (v1.0)1

2.  Concept of Exceptions ◦ Similar to other object languages  Exceptions are thrown and caught ( throw, catch )  Exception is an object of class Exception or derived class ◦ Used for reporting errors  Especially from deeply nested code ◦ Note that …  Uncaught exception causes Fatal Error  Destructors should not throw exceptions  Throwing-catching process is somewhat slow 19. 3. 2015 by Martin Kruliš (v1.0)2

3.  Try-catch Blocks ◦ Exception-safe code is wrapped in try block ◦ First matching catch block handles the exception  Exception is matched by its class try {... Code that may throw an exception... } catch (MyException $e) {... My exception handler... } catch (Exception $e) {... Generic exception handler... } 19. 3. 2015 by Martin Kruliš (v1.0)3

4.  Throwing Exceptions ◦ Keyword throw followed by the exception object throw new Exception('Error …'); ◦ It can be used inside catch-block to re-throw already caught exception try {... } catch (MyException $e) {... Partial exception handling... throw $e; // re-throwing exception } 19. 3. 2015 by Martin Kruliš (v1.0)4

5.  Creating Custom Exceptions ◦ Exception class is derived from Exception ◦ Derived classes need not to override any methods  If the constructor is overridden, parent constructor must be invoked  It is recommended to redefine __toString() ◦ When to customize…  To distinguish a new error type  To add additional data to the exception object 19. 3. 2015 by Martin Kruliš (v1.0)5

6.  Internal Errors ◦ Should not be displayed to the user  User cannot understand them, nor counteract them  Potential security breach ◦ Save them for admin (log, e-mail, …)  Error Control Operator ◦ Symbol @ prepended to an expression ◦ All error messages from the expression are ignored ◦ Suitable for specific local solutions only $data = @file('data_file.txt') or die('Error…'); 19. 3. 2015 by Martin Kruliš (v1.0)6

7.  Error Levels ◦ Define the severity of the error  E_ERROR – fatal errors, terminate the script  E_WARNING – severe errors, but recoverable  E_NOTICE – unusual situations (possible error)  E_USER_xxx – user level error, warning, or notice  E_STRICT – suggestion for improvement ◦ The log can filter selected error levels  Controlled in php.ini or by error_reporting() ◦ User errors can be triggered manually  trigger_error() 19. 3. 2015 by Martin Kruliš (v1.0)7

8.  Setting Up Error Handling ◦ set_error_handler()  Sets a callback that handles runtime errors  Except for Fatal Errors  Can yield handling back to default control  By returning false from the callback function  Default control acts according to php.ini settings ◦ set_exception_handler()  Sets generic handler for uncaught exceptions ◦ restore_error_handler(), restore_exception_handler()  Restore original error handling control 19. 3. 2015 by Martin Kruliš (v1.0)8

9.  Information about Errors ◦ The handling callback gets error level and position ◦ debug_backtrace() – retrieves current call stack ◦ debug_print_backtrace() – prints call stack ◦ Might be wise to log $_GET, $_POST, $_COOKIES, …  PHP Supported Error Logging ◦ error_log() – logs error internally ◦ Error message can be written to the web server log, sent as e-mail, or written to a file or a TCP channel 19. 3. 2015 by Martin Kruliš (v1.0)9 Example

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