PHP Exception Handling

PHP Exception Handling

An exception (Exception) is used to change the normal process of a script when a specified error occurs.

What is an exception?

PHP 5 provides a new approach to object-oriented error handling.

Exception handling is used to change the normal process of a script when a specified error (exception) condition occurs. This condition is called an exception.

When an exception is triggered, it usually occurs:

• Current code state is saved
• Code execution is switched to the pre-defined exception handler function
• Depending on the situation, the processor may start executing the code again from the saved code state, terminating the script execution, or continuing the script from another location in the code

We will show you different error handling methods:

• Basic use of exceptions
• Creating a custom exception handler
• Multiple exceptions
• To re-throw an exception
• Setting the top-level exception handler

Basic use of exceptions

When the exception is thrown, the subsequent code does not continue, and PHP tries to find a matching "catch" block.

If the exception is not captured and does not use Set_exception_handler () for appropriate processing, a serious error (fatal error) will occur, and an error message "Uncaught exception" (uncaught exception) is output.

Let's try to throw an exception without catching it:

<?php//create function with an exceptionfunction checknum ($number) {if ($number >1)  {  throw new Exception ( "Value must be 1 or below");  } return true; }//trigger Exceptionchecknum (2);? >

The above code will get an error like this:

Fatal error:uncaught Exception ' exception ' with message ' Value must being 1 or below ' in C:\webfolder\test.php:6 Stack trace : #0 C:\webfolder\test.php: Checknum #1 {main} thrown in C:\webfolder\test.php on line 6

Try, throw, and catch

To avoid the error in the above example, we need to create the appropriate code to handle the exception.

The correct processing procedure should include:

1. Try-the function that uses the exception should be in the "try" code block. If no exception is triggered, the code will continue to execute as usual. However, if an exception is triggered, an exception is thrown.
2. Throw-this specifies how the exception is triggered. Each "throw" must correspond to at least one "catch"
3. Catch-the "catch" code block catches an exception and creates an object that contains the exception information

Let's trigger an exception:

<?php//creates a function that throws an exception Checknum ($number) {if ($number >1)  {  throw new Exception ("Value must be 1 or Below ");  } return true;  }//triggers an exception in the "try" code block try {checknum (2);//if the exception is thrown and this text won't is shown echo ' If you see this, the Number is 1 or below '; }//catch Exception catch (Exception $e) {echo ' Message: '. $e->getmessage ();}? >

The above code will get an error like this:

Example Explanation:

The above code throws an exception and captures it:

1. Create the Checknum () function. It detects if the number is greater than 1. If it is, an exception is thrown.
2. Call the Checknum () function in the "Try" code block.
3. The exception in the Checknum () function is thrown
4. The catch code block receives the exception and creates an object ($e) that contains the exception information.
5. Output the error message from this exception by calling $e->getmessage () from this exception object

However, to follow the principle that each throw must correspond to a catch, you can set up a top-level exception handler to handle the missing error.

To create a custom Exception class

Creating a custom exception handler is straightforward. We have simply created a special class that can call its function when an exception occurs in PHP. The class must be an extension of the exception class.

This custom exception class inherits all the properties of the PHP exception class, and you can add custom functions to it.

We started to create the exception class:

<?phpclass 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;  } } $email = "[email protected]"; try {//check if  if (Filter_var ($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 ();}? >

This new class is a copy of the old exception class, plus the ErrorMessage () function. Because it is a copy of the old class, it inherits properties and methods from the old class, and we can use the methods of the exception class, such as GetLine (), GetFile (), and GetMessage ().

Example Explanation:

The above code throws an exception and captures it through a custom exception class:

1. The Customexception () class is created as an extension of the old exception class. So it inherits all the properties and methods of the old class.
2. Create the ErrorMessage () function. If the e-mail address is not valid, the function returns an error message
3. Set $email variable to an illegal e-mail address string
4. Execute "Try" code block, because the e-mail address is not valid, so throw an exception
5. Catch code block catches an exception and displays an error message

Multiple exceptions

You can use multiple exceptions for a script to detect multiple situations.

You can use multiple if: else code block, or a switch code block, or nested multiple exceptions. These exceptions can use different exception classes and return different error messages:

<?phpclass 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;}} $email = "[email protected]"; try {//check if  if (Filter_var ($email, filter_validate_email) = = = FALSE)  {  // Throw exception if email is not valid  throw new Customexception ($email),  //check for ' example ' in mail address I F (Strpos ($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 Explanation:

The above code tests two conditions, and throws an exception if any of the conditions are not true:

1. The Customexception () class is created as an extension of the old exception class. So it inherits all the properties and methods of the old class.
2. Create the ErrorMessage () function. If the e-mail address is not valid, the function returns an error message.
3. Executes a "try" block of code that, under the first condition, does not throw an exception.
4. Because e-mail contains the string "Example", the second condition triggers an exception.
5. Catch code block catches exceptions and displays appropriate error messages

If the customexception is not captured and the base exception is tightly captured, the exception is handled there.

To re-throw an exception

Sometimes, when an exception is thrown, you might want to handle it in a different way from the standard. You can throw an exception again in a "catch" code block.

The script should hide the system error from the user. For programmers, system errors may be important, but users are not interested in them. To make it easier for users to use, you can again throw an exception with a friendly message to the user:

<?phpclass Customexception extends Exception {public Function errormessage ()  {  //error message  $ ErrorMsg = $this->getmessage (). ' is not a valid e-mail address. ';  return $ERRORMSG;  } } $email = "[email protected]"; try {  //check for "example" in mail address  if (Strpos ($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 Explanation:

The above code detects if the message address contains the string "example". If there is, throw the exception again:

1. The Customexception () class is created as an extension of the old exception class. So it inherits all the properties and methods of the old class.
2. Create the ErrorMessage () function. If the e-mail address is not valid, the function returns an error message.
3. Set the $email variable to a valid e-mail address, but contains the string "example".
4. The "Try" code block contains another "try" code block so that you can throw the exception again.
5. The exception is triggered because e-mail contains the string "example".
6. Catch catches the exception and re-throws "Customexception".
7. "Customexception" is captured and an error message is displayed.

If an exception is not caught in its current "try" code block, it looks for a catch code block at a higher level.

Setting the top level exception processor (top Exception Handler)

The Set_exception_handler () function sets a user-defined function to handle all uncaught exceptions.

<?phpfunction myexception ($exception) {echo "<b>Exception:</b>", $exception->getmessage ();} Set_exception_handler (' myexception '); throw new Exception (' uncaught exception occurred ');? >

The output of the above code should look like this:

Exception:uncaught Exception occurred

In the above code, there is no "catch" code block, but the top-level exception handler is triggered. You should use this function to catch all uncaught exceptions.

Rules for exceptions

• The code that requires exception handling should be placed inside a try code block to catch a potential exception.
• Each try or throw code block must have at least one corresponding catch code block.
• You can use multiple catch blocks to catch different kinds of exceptions.
• You can throw (re-thrown) exceptions again in a catch code block within a try code block.

In short: If an exception is thrown, it must be captured.

PHP Exception Handling