Exception Handling in PHP

An exception is 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 following specialized keywords for this purpose.

• try: It represent block of code in which exception can arise.
• catch: It represent 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 catch block or after catch block basically it is put for cleanup activity in PHP code.

Why Exception Handling in PHP ?
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 if else block to handle errors. These conditions and code to handle errors got mixed so that becomes unreadable. With try Catch block code becomes readable.
• Grouping of error types: In PHP both basic types and objects can be thrown as exception. It can create a hierarchy of exception objects, group exceptions in namespaces or classes, categorize them according to types.

Exception handling in PHP:

• Following code explains the flow of normal try catch block in PHP:
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  <?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 rised demo(5);    // Exception will be rised here demo(0); ?>

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  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
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  <?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 rised demo(5);    // Exception will be rised here demo(0); ?>

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  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
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  <?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); ?>

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  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 function to all uncaught exception.
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  <?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 rised demo(5);    // Exception will be rised here demo(0);    // Uncaught Exception demo (-3); ?>

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  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
  

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