Parent的类静态变量为子类的不同值

Please, sorry for my English.

My problem:

abstract class Entity
{
    protected static $fieldNames;

    public static function getFieldsNames()
    {
        if (is_null(static::$fieldNames)) {
            foreach (static::$fieldsMap as $name => $map) {
                static::$fieldNames[] = $name;
            }
        }

        return static::$fieldNames;
    }
}

class User extends Entity
{
    protected static $fieldsMap = [
        'id' => [
            // ...
        ],
        'name' => [
            // ...
        ],
        'phone' => [
            // ...
        ]
    ];
}

class Car extends Entity
{
    protected static $fieldsMap = [
        'id' => [
            // ...
        ],
        'brand' => [
            // ...
        ],
        'color' => [
            // ...
        ]
    ];
}

print_r(User::getFieldsNames());
// ['id', 'name', 'phone'] - On first call it works as expected, but...

print_r(Car::getFieldsNames());
// ['id', 'name', 'phone'] :(

If I declare $fieldNames in User and Car classes work fine, but in real project I has tens of static variables such $fieldNames and hundreds of entity's

Is it possible to best solution?

Maybe create small repository class that will keep these static variables by entity's id? or another elegant way?

Thanks any Help!

$fieldNames is static so it's associated with the class itself and not with a specific object. The class in this instance is "Entity". Once you set it it is no longer null.