PHP定义常量声明

I do not understand how this code work. displaying code below , in this code i defined a constant and the value is 10 but it display the output "constant is not defined".

why?

define('HELLO', 10);        
if(defined(HELLO))        
{        
    echo "Constant is defined";        
}        
else        
{        
    echo "Constant is not defined";        
}        

?>

change:

defined(HELLO)

to:

defined('HELLO')

http://php.net/manual/en/function.defined.php