var hoge = [
    {a: 3.6, b: 9.7},
    {a: 5.6, b: 3.7},
    {a: 3.5, b: 1.7}
  ];

str = hoge.join(',');
console.log(str); //[object Object],[object Object],[object Object]

var hoge = [
    {a: 3.6, b: 9.7},
    {a: 5.6, b: 3.7},
    {a: 3.5, b: 1.7}
  ];
var hoge_json = JSON.stringify(hoge);

$json_data = json_decode($_REQUEST['hoge_json']);
↑
array(
    0 => array("a" => "3.6", "b" => "9.7"),
    1 => array("a" => "5.6", "b" => "3.7"),
    2 => array("a" => "3.5", "b" => "1.7")
)