致命错误:访问未声明的静态属性:StaticMethod::$a

Fatal error: Access to undeclared static property: StaticMethod::$a

class StaticMethod
{
public $a=10;
public $b=20;
    public static function sum(){
        return (self::$a+self::$b);
    }
}
echo StaticMethod::sum();

您将sum方法声明为静态，但没有将变量$a和$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