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PHP Exception Handling
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:
- 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:
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 retrieves 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.