use implode and ajax to insert checkbox values

i want to insert my checkbox values, i dont know how to take all the values using ajax and use implode to insert them in one row.

// this is my javascript where i take the data ,

function saveData() {
  var modsubj = $('#modalsubject').val();
  var modsect = $('#modalsection').val();
  var modday = $('#modalday').val();
  var modstart = $('#modalstarttime').val();
  var modend = $('#modalendtime').val();
  var moduser = $('#userID').val();

  $.ajax({
    type: "POST",
    url: "modal.funcs.php?p=add",
    data: "subj=" + modsubj + "&sect=" + modsect + "&day=" + modday + "&start=" + modstart + "&end=" + modend + "&user=" + moduser
  });

}

<script src="http://ift.tt/1oMJErh"></script>
<div class="checkbox">
  <label class="checkbox-inline">
    <input type="checkbox" id="modalday[]" name="modalday[]" value="M">Monday &nbsp;&nbsp;
  </label>
</div>
<div class="checkbox">
  <label class="checkbox-inline">
    <input type="checkbox" id="modalday[]" name="modalday[]" value="T">Tuesday &nbsp;&nbsp;
  </label>
</div>
<div class="checkbox">
  <label class="checkbox-inline">
    <input type="checkbox" id="modalday[]" name="modalday[]" value="W">Wednesday &nbsp;&nbsp;
  </label>
</div>
<div class="checkbox">
  <label class="checkbox-inline">
    <input type="checkbox" id="modalday[]" name="modalday[]" value="Th">Thursday &nbsp;&nbsp;
  </label>
</div>
<div class="checkbox">
  <label class="checkbox-inline">
    <input type="checkbox" id="modalday[]" name="modalday[]" value="F">Friday &nbsp;&nbsp;
  </label>
</div>

this is my php function, i used the implode function so that i can insert the data on one row.

$page = isset($_GET['p'])?$_GET['p']:'';
  if($page=='add'){
      foreach ($_POST['day'] as $key => $value) {
        $subj = $_POST['subj'];
        $sect = $_POST['sect']; 
        $day = implode("",$_POST['day']); 
        $strTime = $_POST['start'];
        $endTime = $_POST['end'];
        $user_id = $_POST['user'];
      }

        $auth_user->createSchedule($subj,$sect,$day,$strTime,$endTime,$user_id);
        $schedRow = $auth_user->readSchedule();

  } else if ($page=='edit') {

  }