php - What is $previous in Exception class? -

what $previous in exception constructor argument denote? how use it?

    class myexception extends \exception {      private $message;     private $code;       public function __construct($message,$code,\exception $previous=null){         $this->message = $message;         $this->code = isset($code) ? $code : 0;         parent::__construct($message,$code,$previous);           }        } 

i didn't find in api doc

if throw exception because caught exception, can add original exception $previous. means can "nest" exceptions:

try {     throw new fooexception('foo exception'); } catch (fooexception $e) {     $code = 1;     throw new barexception('bar exception', $code, $e); } 

you can loop on exception "stack" instead of exception caught, providing more context.

while($e instanceof \exception) {     echo $e->getmessage();     $e = $e->getprevious(); } 

now, use if you're implementing library can throw exception, you'd wrap exception in own. way, clients' code needs know of your exceptions, , not of exceptions dependencies of code has, while not losing kind of information.