What is an Exception
Exception handling is used to change the normal flow of the code execution if a specified error (exceptional) condition occurs. This condition is called an exception.
This is what normally happens when an exception is triggered:
The current code state is saved
The code execution will switch to a predefined (custom) exception handler function
Depending on the situation, the handler may then resume the execution from the saved code state, terminate the script execution or continue the script from a different location in the code
Basic Use of Exceptions
When an exception is thrown, the code following it will not be executed, and PHP will try to find the matching "catch" block.
If an exception is not caught, a fatal error will be issued with an "Uncaught Exception" message.
Lets try to throw an exception without catching it:
<?php
//create function with an exception
function checkNum($number) {
if($number>1) {
throw new Exception("Value must be 1 or below");
}
return true;
}
//trigger exception
checkNum(2);
?>
The code above will get an error like this:
Fatal error: Uncaught exception 'Exception'
with message 'Value must be 1 or below' in C:\webfolder\test.php:6
Stack trace: #0 C:\webfolder\test.php(12):
checkNum(28) #1 {main} thrown in C:\webfolder\test.php on line 6
Try, throw and catch
To avoid the error from the example above, we need to create the proper code to handle an exception.
Proper exception code should include:
Try - A function 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
Lets try to trigger an exception with valid code:
<?php
//create function with an exception
function checkNum($number) {
if($number>1) {
throw new Exception("Value must be 1 or below");
}
return true;
}
//trigger exception in a "try" block
try {
checkNum(2);
//If the exception is thrown, this text will not be shown
echo 'If you see this, the number is 1 or below';
}
//catch exception
catch(Exception $e) {
echo 'Message: ' .$e->getMessage();
}
?>
The code above will get an error like this:
Message: Value must be 1 or below
Example explained:
The code above throws an exception and catches it:
The checkNum() function is created. It checks if a number is greater than 1. If it is, an exception is thrown
The checkNum() function is called in a "try" block
The exception within the checkNum() function is thrown
The "catch" block retrives the exception and creates an object ($e) containing the exception information
The error message from the exception is echoed by calling $e->getMessage() from the exception object
However, one way to get around the "every throw must have a catch" rule is to set a top level exception handler to handle errors that slip through.
Creating a Custom Exception Class
Creating a custom exception handler is quite simple. We simply 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 add custom functions to it.
Lets create an exception class:
<?php
class customException extends Exception {
public function errorMessage() {
//error message
$errorMsg = 'Error on line '.$this->getLine().' in '.$this->getFile()
.': <b>'.$this->getMessage().'</b> is not a valid E-Mail address';
return $errorMsg;
}
}
$email = "someone@example...com";
try {
//check if
if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE) {
//throw exception if email is not valid
throw new customException($email);
}
}
catch (customException $e) {
//display custom message
echo $e->errorMessage();
}
?>
The new class is a copy of the old exception class with an addition of the errorMessage() function. Since it is a copy of the old class, and it inherits the properties and methods from the old class, we can use the exception class methods like getLine() and getFile() and getMessage().
Example explained:
The code above throws an exception and catches it with a custom exception class:
The customException() class is created as an extension of the old exception class. This way it inherits all methods and properties from the old exception class
The errorMessage() function is created. This function returns an error message if an e-mail address is invalid
The $email variable is set to a string that is not a valid e-mail address
The "try" block is executed and an exception is thrown since the e-mail address is invalid
The "catch" block catches the exception and displays the error message
ref:http://www.w3schools.com/php/php_exception.asp
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