I have declared a private variable to hold a custom post type arguments array. I can't understabd why when I try to activate the plugin I get always this error: Constant expression contains invalid operations.

Is there a fix?

code example

<?php
class CustomPostType {

public $services = array( ... );


 public function init()
 {
  register_post_type( 'services', $this->services );
 }

}
?>