提交表单后，选项中的未定义索引

Question

技能是表单中的选项标记，但未选中的选项在进程页面中提交后显示未定义的索引。这是我在流程页面中使用的代码：

if (isset($_POST['submit_form'])) {
    $name    = strip_tags($_POST['name']);
    $email   = strip_tags($_POST['email']);
    $subject = htmlspecialchars($_POST['subject']);
    $gender  = strip_tags($_POST['gender']);
    $country = strip_tags($_POST['country']);
    $comment = trim(htmlspecialchars($_POST['comment']));
    $skills1 = $_POST['skills1'];
    $skills2 = $_POST['skills2'];
    $skills3 = $_POST['skills3'];
    $skills4 = $_POST['skills4'];
    $skills5 = $_POST['skills5'];

    $ins_sql = "INSERT INTO comments (name, email, subject, gender, skills1, skills2, skills3, skills4, skills5,  country, comment) VALUES ('$name', '$email', '$subject', '$gender', '$skills1', '$skills2', '$skills3', '$skills4', '$skills5', '$country', '$comment')";
    $run_sql = mysqli_query($conn, $ins_sql);
} else {

}