PHP Constant doesn't exist: Notice

echo $non_existing_variable; // Error
echo NON_EXISTING_CONSTANT; // Now also an error

if (defined('CONSTANT')) {
    echo CONSTANT;
} else {
    throw new Exception('Undefined Constant.');
}

class Config{
    const CONFIG_ONE = 42;
}
//will produce a fatal error
if(Config::CONFIG_TWO){
   //...
}