How to insert to the mysql array object - php

Question

I'm new to Rest api,I'm developing a student management system using rest api and angular js.When I registering students from student registration form there is a ng-grid to select courses of students going attend in the institute,
I loaded selected courses in an angular object like this.
var Course = {
"std_id": "1",
"course_id": $scope.studentCourse,
"started_date": $scope.startedDate
};
in this courses object courses are selected values from ng-grid plus student and registrations date.My problem is I have loaded objects to an object and Im stuck with process this obejects in side my restfull api.
my json object pass like this
[{
"std_id":"1",
"courses":
[{"course_id":"2"},
{"course_id":"4"},
{"course_id":"8"}],
"started_date":"2015-9-18"
}]
Here my php rest call
private function InsertStudentCourse() {
if ($this->get_request_method() != "POST") {
$this->response('', 406);
}
$course= json_decode(file_get_contents("php://input"), true);
$column_names = array ('std_id','course_id','started_date');
$columns = '';
$values = '';
$keys = array_keys($course);
//$this->response($this->json($deviceTypeId), 200);
foreach ($column_names as $desired_key) { // Check the Department received. If blank insert blank into the array.
if (!in_array($desired_key, $keys)) {
$$desired_key = '';
} else {
$$desired_key = $course[$desired_key];
}
$columns = $columns . $desired_key . ',';
$values = $values . "'" . $$desired_key . "',";
}
$query = "INSERT INTO student_course (" . trim($columns, ',') . ") VALUES(" . trim($values, ',') . ")";
//$query = "INSERT INTO student_course ('course_id') VALUES ($c)";
if (!empty($course)) {
$r = $this->mysqli->query($query) or die($this->mysqli->error . __LINE__);
$success = array('status' => "Success", "msg" => "student Course Type Created Successfully.", "data" => $course);
$this->response($this->json($success), 200);
} else {
$this->response('', 204);
} //"No Content" status
}
I want to insert this object array like this
std_id course_id started_date
- 1 , 2, 2015-9-18
- 1 , 4, 2015-9-18
- 1 , 8, 2015-9-18
how can I get this done