无法从php脚本获取记录到Android应用程序

I am using android volley library to get the records from database on server. I send a query from my android app to this php script. When I write query myself in 4th line as follows $myquery = "Select firstname from Student;"; then all results are returned correctly but when I send a query directly, it doesn't returns records. I am sure that the issue is in php script, please help me where I am going wrong in this basic script?

function showStudent()
{
    global $connect;
    $myquery = $_POST["query"];

    $result = mysqli_query($connect,$myquery);
    $number_of_rows = mysqli_num_rows($result);

    $temp_array = array();

    if($number_of_rows>0)
    {
        while( $row = mysqli_fetch_assoc($result) )
        {
            $temp_array[] = $row;
        }
    }

    header('Content-Type: application/json');
    echo json_encode(array("dataArray"=>$temp_array));
    mysqli_close($connect);
}
?>

Change to this

$myquery = $_REQUEST['query'];

Instead of this

$myquery = $_POST["query"];