$param_value = ${$param_name};是什么意思？

mine

1. function get_session($param_name)
   
2. {
   
3.   global $HTTP_POST_VARS;
   
4.   global $HTTP_GET_VARS;
   
5.   global ${$param_name};
   
6. 
   
7.   $param_value = "";
   
8.   if(!isset($HTTP_POST_VARS[$param_name]) && !isset($HTTP_GET_VARS[$param_name]) && session_is_registered($param_name))
   
9.     $param_value = ${$param_name};
   
10. 
    
11.   return $param_value;
    
12. }

复制代码

上面代码中的$param_value = ${$param_name};是什么意思？