Exception Handling in PHP

An exception is an unexpected program result that can be handled by the program itself. Exception Handling in PHP is almost similar to exception handling in all programming languages. 

PHP provides the following specialized keywords for this purpose.

• try: It represents a block of code in which exceptions can arise.
• catch: It represents a block of code that will be executed when a particular exception has been thrown.
• throw: It is used to throw an exception. It is also used to list the exceptions that a function throws, but doesn’t handle itself.
• finally: It is used in place of a catch block or after a catch block basically it is put for cleanup activity in PHP code.

Why Exception Handling in PHP?

The following are the main advantages of exception handling over error handling

• Separation of error handling code from normal code: In traditional error handling code there is always an if-else block to handle errors. These conditions and code to handle errors got mixed so that became unreadable. With try Catch block code becomes readable.
• Grouping of error types: In PHP both basic types and objects can be thrown as exceptions. It can create a hierarchy of exception objects, group exceptions in namespaces or classes, and categorize them according to types.

Exception handling in PHP:

• The following code explains the flow of normal try-catch block in PHP: 

php

<?php

// PHP Program to illustrate normal
// try catch block code
function demo($var) {
    echo " Before try block";
    try {
        echo "\n Inside try block";

        // If var is zero then only if will be executed
        if($var == 0)
        {

            // If var is zero then only exception is thrown
            throw new Exception('Number is zero.');

            // This line will never be executed
            echo "\n After throw (It will never be executed)";
        }
    }

    // Catch block will be executed only
    // When Exception has been thrown by try block
    catch(Exception $e) {
            echo "\n Exception Caught", $e->getMessage();
        }

        // This line will be executed whether
        // Exception has been thrown or not
        echo "\n After catch (will be always executed)";
}

// Exception will not be raised
demo(5);

// Exception will be raised here
demo(0);
?>

Output:

 Before try block
 Inside try block
 After catch (will be always executed)
 Before try block
 Inside try block
 Exception CaughtNumber is zero.
 After catch (will be always executed)

• Following code explains the flow of normal try catch and finally block in PHP 

php

<?php

// PHP Program to illustrate normal
// try catch block code
function demo($var) {

    echo " Before try block";
    try {
        echo "\n Inside try block";

        // If var is zerothen only if will be executed
        if($var == 0) {

            // If var is zero then only exception is thrown
            throw new Exception('Number is zero.');

            // This line will never be executed
            echo "\n After throw it will never be executed";
        }
    }

    // Catch block will be executed only
    // When Exception has been thrown by try block
    catch(Exception $e) {
        echo "\n Exception Caught" . $e->getMessage();
    }
    finally {
        echo "\n Here cleanup activity will be done";
    }

    // This line will be executed whether
    // Exception has been thrown or not
    echo "\n After catch it will be always executed";
}

// Exception will not be raised
demo(5);

// Exception will be raised here
demo(0);
?>

Output:

 Before try block
 Inside try block
 Here cleanup activity will be done
 After catch (will be always executed)
 Before try block
 Inside try block
 Exception CaughtNumber is zero.
 Here cleanup activity will be done
 After catch (will be always executed)

• Using Custom Exception Class 

php

<?php
class myException extends Exception {
    function get_Message() {

        // Error message
        $errorMsg = 'Error on line '.$this->getLine().
                    ' in '.$this->getFile()
        .$this->getMessage().' is number zero';
        return $errorMsg;
    }
}

function demo($a) {
    try {

        // Check if
        if($a == 0) {
            throw new myException($a);
        }
    }

    catch (myException $e) {

        // Display custom message
        echo $e->get_Message();
    }
}

// This will not generate any exception
demo(5);

// It will cause an exception
demo(0);
?>

Output:

Error on line 20 in /home/45ae8dc582d50df2790517e912980806.php0 is number zero

• Set a Top Level Exception Handler: The set_exception_handler() function set all user-defined functions to all uncaught exceptions. 

php

<?php

// PHP Program to illustrate normal
// try catch block code

// Function for Uncaught Exception
function myException($exception) {

    // Details of Uncaught Exception
    echo "\nException: " . $exception->getMessage();
}

// Set Uncaught Exception handler
set_exception_handler('myException');
function demo($var) {

    echo " Before try block";
    try {
        echo "\n Inside try block";

        // If var is zero then only if will be executed
        if($var == 0)
        {

            // If var is zero then only exception is thrown
            throw new Exception('Number is zero.');

            // This line will never be executed
            echo "\n After throw (it will never be executed)";
        }
    }

    // Catch block will be executed only
    // When Exception has been thrown by try block
    catch(Exception $e) {
        echo "\n Exception Caught", $e->getMessage();
    }

    // This line will be executed whether
    // Exception has been thrown or not
    echo "\n After catch (will be always executed)";

    if($var < 0) {

        // Uncaught Exception
        throw new Exception('Uncaught Exception occurred');
    }
}

// Exception will not be raised
demo(5);

// Exception will be raised here
demo(0);

// Uncaught Exception
demo (-3);
?>

Output:

 Before try block
 Inside try block
 After catch (will be always executed)
 Before try block
 Inside try block
 Exception CaughtNumber is zero.
 After catch (will be always executed)
 Before try block
 Inside try block
 After catch (will be always executed)
 Exception: Uncaught Exception occurred