How to create a DOMDocument object variable in the class? What should I do?

How to create a DOMDocument object variable in the class? & Lt ;? PhpclassXXX {private $ _ music = newDOMDocument ();} in this way, an error is reported. if this is not the case in the class, $ _ music = newDOMDocument (); how to create a DOMDocument object variable in the class?
Class XXX
{
Private $ _ music = new DOMDocument ();
}

In this way, an error is reported,
However, this is not the case in the class.
$ _ Music = new DOMDocument ();

No error is reported. how can I create such an object variable in the class?

------ Solution --------------------
A syntax error occurs. variables from the manual attributes can be initialized, but the initialization value must be a constant. the constant here means that the php script is a constant during compilation, instead of the constant calculated in the running phase after the compilation phase.
Generally, values are assigned to the _ construct () constructor, while values are declared outside the construct () constructor.