It's work

class Coba{
    function a(){
        function b(){
            return 'ABC';
        }
        return b();
    }
}

$a = new Coba();
$a->a();

but it's not work

class Coba{
    public function a(){
        public function b(){
            return 'ABC';
        }
        return b();
    }
}

$a = new Coba();
$a->a();

Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions.  Due to the volume
of reports we can not explain in detail here why your report is not
a bug.  The support channels will be able to provide an explanation
for you.

Thank you for your interest in PHP.

.