提交正确的值后，单选按钮不会保持检查状态

I cannot understand why the radio button does not stay checked after submitting the right value to the database. I can see in the database the value being updated correctly but the corresponded radio button is empty. Could you help me?

 <?php

session_start();

include 'connect_to_database.php';
$tbl_name="members"; // Table name
$db = connect_to_database();

$email = $_SESSION['email'];

// To protect MySQL injection (more detail about MySQL injection)

function add_value_to_members($db, $email, $key, $value)
{
    $new_name=$_POST[$value];
    $new_name = stripslashes($new_name);
    $new_name = mysqli_real_escape_string($db, $new_name);
    if (strlen($new_name) > 0){
        $new_name_sql="UPDATE members SET $key='$new_name' WHERE email='$email'";
        $result=mysqli_query($db, $new_name_sql);
        echo $result;
    }
}

add_value_to_members($db, $email, "gender", "new_gender");

header("Location: ../edit_user.php");
exit();

// html

<div class="form-group">
    <label class="col-lg-3 control-label">Gender</label>
    <div class="col-lg-8">
        <div class="radio">
            <label>
                <input type="radio" name="new_gender" value="male" <?php if (isset($_POST['gender']) && $_POST['gender'] == "male") echo "checked"; ?>> Male
            </label>
        </div>
        <div class="radio">
            <label>
                <input type="radio" name="new_gender" value="female" <?php if(isset($_POST['gender']) && $_POST['gender']=="female" ) echo "checked"; ?>> Female
            </label>
        </div>
    </div>
</div>

Where is your form tag? Anyway i think it has to do with sending a header wich redirects, and doesnt send a post, if this is what you need you could place the radio button value into a session variable right after updating to your database and check in your radio button tag for the session value or you could use a get paramater wich gets sended into the header url