Zend和静态类属性

我试图在定义静态类属性时为其赋值:

namespace Base;

abstract class Skeleton {   

protected static $entityManager = \Zend_Registry::get("EntityManager");
    ...
}

Parse error: syntax error, unexpected '(', expecting ',' or ';' in /var/www/
somewhere/application/models/Base/Skeleton.php on line 6

如果我只给它分配一个简单的字符串值:

protected static $entityManager = "string";

一切都很好。我在做PHP无法处理的事情吗?如果是,如何解决?