$select_comment = $conn->prepare("SELECT * FROM comments where student_id=?");
$select_comment->bind_param("i", $user_id);
$select_comment->execute();

$select_answers = $conn->prepare("SELECT * FROM answers where student_id=$user_id and exercise_time='".$ex_tim."'");
$select_answers->bind_param("is", $user_id, $ex_tim);
$select_answers->execute();

$insert_comment = $conn->prepare("INSERT INTO comments (student_id, comment, time) VALUES(?,?,?)");
$insert_comment->bind_param("iss", $user_id,$comment,$ex_tim);
$insert_comment->execute();

Fatal error: Uncaught Error: Call to a member function bind_param() on boolean