Exceptions are used to change
the normal flow of a script if a specified error occurs
What is an Exception
With PHP 5 came a new object
oriented way of dealing with errors.
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:
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
We will show different error
handling methods:
- Basic use of Exceptions
- Creating a custom exception handler
- Multiple exceptions
- Re-throwing an exception
- Setting a top level exception handler
Note: Exceptions should only be
used with error conditions, and should not be used to jump to another place in
the code at a specified point.
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:
1. 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"
2. Throw - This is how you
trigger an exception. Each "throw" must have at least one
"catch"
3. 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:
1. The checkNum() function is
created. It checks if a number is greater than 1. If it is, an exception is
thrown
2. The checkNum() function is
called in a "try" block
3. The exception within the
checkNum() function is thrown
4. The "catch" block
retrives the exception and creates an object ($e) containing the exception
information
5. 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:
1. 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
2. The errorMessage() function
is created. This function returns an error message if an e-mail address is
invalid
3. The $email variable is set to
a string that is not a valid e-mail address
4. The "try" block is
executed and an exception is thrown since the e-mail address is invalid
5. The "catch" block
catches the exception and displays the error message
Multiple Exceptions
It is possible for a script
to use multiple exceptions to check for multiple conditions.
It is possible to use several
if..else blocks, a switch, or nest multiple exceptions. These exceptions can
use different exception classes and return different error messages:
|
<?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($email); } //check for "example" in mail address if(strpos($email, "example") !== FALSE) { throw new Exception("$email is an example e-mail"); } } catch (customException $e) { echo $e->errorMessage(); } catch(Exception $e) { echo $e->getMessage(); } ?> |
Example explained:
The code above tests two
conditions and throws an exception if any of the conditions are not met:
1. 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
2. The errorMessage() function
is created. This function returns an error message if an e-mail address is
invalid
3. The $email variable is set to
a string that is a valid e-mail address, but contains the string
"example"
4. The "try" block is
executed and an exception is not thrown on the first condition
5. The second condition triggers
an exception since the e-mail contains the string "example"
6. The "catch" block
catches the exception and displays the correct error message
If there was no
customException catch, only the base exception catch, the exception would be
handled there
Re-throwing Exceptions
Sometimes, when an exception
is thrown, you may wish to handle it differently than the standard way. It is
possible to throw an exception a second time within a "catch" block.
A script should hide system
errors from users. System errors may be important for the coder, but is of no
interest to the user. To make things easier for the user you can re-throw the
exception with a user friendly message:
|
<?php
class customException extends Exception { public function errorMessage() { //error message $errorMsg = $this->getMessage().' is not a valid E-Mail address.'; return $errorMsg; } } $email = "someone@example.com"; try { try { //check for "example" in mail address if(strpos($email, "example") !== FALSE) { //throw exception if email is not valid throw new Exception($email); } } catch(Exception $e) { //re-throw exception throw new customException($email); } } catch (customException $e) { //display custom message echo $e->errorMessage(); } ?> |
Example explained:
The code above tests if the
email-address contains the string "example" in it, if it does, the
exception is re-thrown:
1. 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
2. The errorMessage() function
is created. This function returns an error message if an e-mail address is
invalid
3. The $email variable is set to
a string that is a valid e-mail address, but contains the string
"example"
4. The "try" block
contains another "try" block to make it possible to re-throw the
exception
5. The exception is triggered
since the e-mail contains the string "example"
6. The "catch" block
catches the exception and re-throws a "customException"
7. The
"customException" is caught and displays an error message
If the exception is not
caught in its current "try" block, it will search for a catch block
on "higher levels".
Set a Top Level Exception Handler
The set_exception_handler()
function sets a user-defined function to handle all uncaught exceptions.
|
<?php
function myException($exception) { echo "<b>Exception:</b> " , $exception->getMessage(); } set_exception_handler('myException'); throw new Exception('Uncaught Exception occurred'); ?> |
The output of the code above
should be something like this:
|
Exception: Uncaught Exception
occurred
|
In the code above there was no
"catch" block. Instead, the top level exception handler triggered.
This function should be used to catch uncaught exceptions.
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 classes of exceptions
- Exceptions can be thrown (or re-thrown) in a catch block within a try block
A simple rule: If you throw
something, you have to catch it.
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