致命错误：访问未声明的静态属性：StaticMethod :: $ a

Question

class StaticMethod
{

public $a=10;

public $b=20;

    public static function sum(){
        return (self::$a+self::$b);
    }
}

echo StaticMethod::sum();

Answer 1

您已将sum方法声明为静态，但您未将变量$a和$b声明为静态。

class StaticMethod
{
    public static $a = 10;
    public static $b = 20;

    public static function sum() {
        return (self::$a + self::$b);
    }
}

echo StaticMethod::sum(); //returns 30