Error Handling and Exception

Posted 2020-06-17

The default error handling in php is very simple.An error message with filename, line number and a message describing the error is sent to the browser.

PHP has different error handling methods:

• Simple "die()" statements
• Custom errors and error triggers
• Error reporting

 

Using the die() function

<?php

　　if(!file_exists(""a.txt)){　　

　　　　die("File not found");

　　}else{

　　　　$file = fopen("a.txt", "r");

　　}

?>

 

Creating a Custom Error Handler

Creating a custom error handler is quite simple.We simply create a special function that can be called when an error occurs in PHP.

The function must be able to handle a minimun of 2 parameters(error level and message) but can accept up to 5 parameters(optionally: file, line-number, and the error context)

error_function(error_level, error_message, error_file, error_line, error_context);

error_level: required.Specifies the error report level for the user-defined error.Must be a value number.

error_message: required.Specifies the error message for the user-defined error.

error_file: optional.Specifies the filename in which the error occurred.

error_line: optional.Specifies the line number in which the error occurred.

error_context: optional.Specifies an array containing every variable, and their values, in use the error occurred

 

error_levels:

2　　E_WARNING　　Nonn-fatal run-time errors.Execution of the script is not halted

8　　E_NOTICE　　Run-time notices.The script found something that might be an error, but could also happen when running a script normally

256　　E_USER_ERROR　　Fatal user-generated error.This is like an E_ERROR set by the programmer using the PHP function trigger_error()

512　　E_USER_WARNING　　Non-fatal user-generated warning.This is like an E_WARNING set by the programmer using the PHP function trigger_error()

1024　　E_USER_NOTICE　　User-generated notice.This is like an E_NOTICE set by the programmer using the PHP function trigger_error()

4096　　E_RECOVERABLE_ERROR　　Catchable fatal error.This is like an E_ERROR but can be caught by a user define handle

8191　　E_ALL　　All the errors and warnings

 

<?php

　　function customError($errno, $errstr){

　　　　echo "<b>Error:</b>[$errno] $errstr<br>";

　　　　echo "Ending script";

　　　　die();

　　}

?>

 

The default error handler for PHP is the built in error handler.We are going to make the function above the default error handler for the duration of the script.

set_error_handler("customError");

<?php

　　function customError($errno, $errStr){

　　　　echo "<b>Error:</b>[$errno] $errstr<br>";

　　}

　　

　　set_error_handler("cutomError");

　　

　　echo($test); //Error: [8] Undefined variable: test

?>

 

In a script where users can input data it is useful to trigger errors when an illegal input occurs.In PHP, this is done by the trigger_error() function.

<?php

　　$test = 2;

　　if($test > 1){

　　　　trigger_error("Value must be 1 or below");// Notice: Value must be 1 or below

　　}

?>

An error type can be triggered anywhere you wish in a script, and by adding a second parameter, you can specify what error level is triggered.

Possible error types:

• E_USER_ERROR    Fatal user-generated run-time error.Errors that can not be recovered from.Execution of the script is halted
• E_USER_WARNING    Non-fatal user-generated run-time warning.Execution of the script is not halted
• E_USER_NOTICE    Default.User-generated run-time notice.The script found something that might be an error, but could also happen when running a script normally

 

Error Logging

By default, PHP sends an error log to the server‘s logging system or a file, depending on how the error_log configuration is set in the php.ini file.By using the error_log() function you can send error logs to a specified or a remote destination.

 

<?php

　　//error handler function 

　　function customError($errno, $errstr){

　　　　echo "<b>Error:</b> [$errno] $errstr</br>";

　　　　echo "Webmaster has been notified";

　　　　error_log("Error: [$errno] $errstr", 1, "[email protected]", "From: [email protected]");

　　}

 

　　set_error_handler("customError", E_USER_WARNING);

 

　　$test = 2;

　　if($test > 1){

　　　　trigger_error("Value must be 1 or below", E_USER_WARNING);

　　}

?>

 

Exceptions are used to change the normal flow of a script if a specified error occurs.

PHP has different error handling methods:

• Basic use of Exception
• Creating a custom exception handler
• Multiple exceptions
• Re-throwing an exception
• Setting a top level exception handler

 

Basic Use of Exceptions

<?php

　　function checkNum($number){

　　　　if($number > 1){

　　　　　　throw new Exception("Value must be 1 or below");

　　　　}

　　　　return true;

　　}

　　

　　checkNum(2);

?>

 

Try, throw and catch

Proper exception code shoule include:

Try--A fucntion using an exception should be in a "try" block.If the exception does not trigger, the code will continue as normal.However if the exception triggers, an exception is "thrown"

Throw--This is how you trigger an exception.Each "throw" must have at least one "catch"

Catch--A "catch" block retrieves an exception and creates an object containing the exception information

 

<?php

　　function checkNum($number){

　　　　if($number > 1){

　　　　　　throw new Exception("Value must be 1 or below");

　　　　}

　　　　return true;

　　}

 

　　try{

　　　　checkNum(2);

　　　　echo ‘If you see this, the number is 1 or below‘;

　　}

 

　　catch(Exception $e){

　　　　echo ‘Message: ‘  .$e->getMessage();

　　}

?>

 

Creating a Custom Exception Class

To create a custom exception handler you must create a special class with functions that can be called when an exception occurs in PHP.The class must be an extension of the exception class.

The custom exception class inherits the properties from PHP‘s exception class and you can ad customm functions to it.

<?php

　　class customException extends Exception{

　　　　public function errorMessage(){

　　　　　　$errorMsg = ‘Error on line ‘ .$this->getLine().‘  in ‘ .$this->getFile(). ‘:<b>‘ .$this->getMessage(). ‘</b> is not a valid E-Mail address‘ ;

　　　　　　return $errorMsg;

　　　　}

　　}

 

　　$email = "[email protected]";

 

　　try{

　　　　if(filter_var($email, FILTER_VALIDATE_EMAIL) === false){

　　　　　　throw new customException($email);

　　　　}

　　}

 

　　catch(customException $e){

　　　　echo $e->errorMessage();

　　}

?>

 

<?php

　　class customException extends Exception{

　　　　public function errorMessage(){

　　　　　　$errorMsg = ‘Error on line ‘ .$this->getLine(). ‘ in ‘ .$this->getFile() . ‘: <b>‘ .$this->getMessage().‘</b> is not  a valid Email address‘.

　　　　　　return $errorMsg;

　　　　}

　　}

 

　　$email = "[email protected]";

 

　　try{

　　　　if(filter_var($email, FILTER_VALIDATE_EMAIL) === false){　　

　　　　　　throw new customException($email);

　　　　}

　　　　if(strpos($email, "example") !== false){

　　　　　　throw new Exception("$email is an example e-mail");

　　　　}

　　}

　　catch(customException $e){　　

　　　　echo $e -> errorMessage();

　　}

 

　　catch(Exception $e){

　　　　echo $e ->getMessage();

　　}

?>

 

<?php

　　class customException extends Exception{

　　　　public function errorMessage{

　　　　　　$errorMsg =$this->getMessage().‘ is not a valid E-Mail address‘;

　　　　　　return $errorMsg;

　　　　}

　　}

　　

　　$email = "[email protected]";

　　

　　try{

　　　　try{

　　　　　　if(strpos($email, "example") !== false)

　　　　　　　　throw new Exception($email);

　　　　　　}

　　　　}

　　　　catch(Exception $e){

　　　　　　throw new customException($email);

　　　　}

　　}

　　catche(customException $e){

　　　　echo $e->errorMessage();

　　}

?>

 

<?php

　　function myException($exception){

　　　　echo "<b>Exception:</b>" .$exception->getMessage();
　　}

 

　　set_exception_handler(‘myException‘);

 

　　throw new Exception(‘Uncaught Exception occured‘);

?>

 

Rules for exceptions

• Code may be surrounded in a try block, to help catch potential exceptions
• Each try block or "throw" must have at least one corresponding catch block
• Multiple catch blocks can be used to catch different classed of exceptions
• Exceptions can be thrown in a catch block within a try block