无法将sessionStorage数据转换为php中的对象数组

I am trying to get sessionStorage data to a php string variable. Then from there I am trying to convert it to a php array of objects. My target is to save stored data in sessionStorage to a mysql database. Following is the process I am following.

$jsonString = "<script>document.write(sessionStorage['sca'])</script>";
$arr = json_decode($jsonString );
var_dump($arr );      
print_r($arr);

The Output I am getting is NULL

What I want is the following

Array ( [0] => stdClass 
        Object ( [name] => iphone [s] => black [price] => 50000 [id] => 2 [qty] => 3 ) 
        [1] => stdClass 
        Object ( [name] => iphone [s] => black [price] => 50000 [id] => 2 [qty] => 3 ) 
      )

Can't figure what I am doing wrong