In an AJAX example like here you can solve this problem on this way:
.php file (ajax return function)
$myArray = array("object_id"=>1, "object_title"=>"Testobject");
return json_encode($myArray);
.js file (javascript function)
...
if(g_httpRequest.readyState == 4)
{
var jsonRes = JSON.parse(g_httpRequest.responseText);
alert(jsonRes.object_title)
}
...