PHP单页提交失败，没有错误

php单次提交失败，页面保持不变，数据也未添加到数据库中。当我单击“取消”按钮时，它将重定向到一个新表单，因为它应该可以工作。

如何使提交工作正常并将表单数据添加到数据库？下面是php和表单html代码，并附带了表单图片

<div class="form-group">
                <label for="s_fname">First name</label> *
                <input type="text" class="form-control" placeholder="First name"> 
                <label for="s_lname">Last name</label> *
                <input type="text" class="form-control" placeholder="Last name"> 
                <label for="s_mname">Middle name</label>
                <input type="text" class="form-control" placeholder="Middle name">
                <label for="s_dob">Date of Birth</label> *
                <input type="text" class="form-control" placeholder="DD/MM/YYYY"> 
                <label for="g_phone">Phone Number</label> *
                <input type="text" class="form-control" placeholder="Phone number"> 
                <label for="g_email">Email</label> *
                <input type="text" class="form-control" placeholder="Email address"> 
                <label for="entry_year">Entry level</label> *
                <input type="text" class="form-control" placeholder="Entry level"> 
            </div>

            <div class="row text-right">
                <input type="submit" name="submit" value="Save" class="btn btn-default" /> 
                &nbsp 
                <button type="button" class="btn btn-default" aria-label="Left Align"><a href="new_student.php">Cancel</a></button>
            </div>

<?php 
if (isset($_POST['submit'])) {
    # process the form

    $s_fname = $_POST["s_fname"];
    $s_lname = $_POST["s_lname"];
    $s_mname = $_POST["s_mname"];
    $s_dob = $_POST["s_dob"];
    $g_phone = $_POST["g_phone"];
    $g_email = $_POST["g_email"];
    $entry_year = $_POST["entry_year"];

    $query  = "insert into students (s_fname, s_lname, s_mname, s_dob, g_phone, g_email, entry_year) ";
    $query .= "values ('{$s_fname}', '{$s_lname}', '{$s_mname}', {$s_dob}, {$g_phone}, {$g_email}, {$entry_year})";

    $result = mysqli_query($connection, $query);

    if ($result) {
        # successful
        redirect_to("student.php");
    } else {
        # failure
        redirect_to("new_student.php");
    }
}
?>