Parse error: syntax error... When I try get one static instance another php class

I'm trying to get a static instance of a class in another php class (singleton pattern), but it throws a syntax error "Parse error: syntax error, unexpected '(', waiting ',' or ';'".Can someone explain me why I can not do that in Php?. I am using php 5.5.38.

Example code:

<?php

    Class A{

        private static $instance=null;

        private function __Constrcut(){
          #Some initialization of parameters

       }
       public static function getInstance(){
          if(self::$instance==null){
             self::$instance=new A();
          }
          return self::$instance;
       }
   }
?>

Another class file

<?php

    include ('path/to/A class/A.php');

    Class B{

       #Line error
       private static $a_instance = A::getInstance();

       public funtion __Construct(){
          #Some parameters....           

       }

      #Some other functions....

  }

?>

Thank you for helpme :) .