在静态属性中不能使用静态方法？ [语法错误，意外'（']

I have a Config class with:

class Config{
    [...]
    public static function url()
    {
        if(self::debug)
            return "https://localhost:44300";
        else
            return "https://www.mysite.com";
    }
    [...]
}

then a class to manage Facebook logins, where I want to define a string with the callback uri:

class Fb
{
    public static $login_redirecturi = Config::url() . "/login/";
    [...]
}

But I can't understand why it gives an error:

Parse error: syntax error, unexpected '(', expecting ',' or ';' in [...] on line 20

How can I do?

You can't call methods on property declarations.

From the PHP docs:

This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

http://php.net/manual/en/language.oop5.properties.php