我有一个数组: $countries = array( "af" => "Afghanistan", "ax" => "Åland Islands", "al" => "Albania", "dz" => "Algeria" ); 还有一些变量: $as = "American Samoa"; $ad = "Andorra"; 我该如何将变量合并到数组中,同时保持变量名作为数组的键?