Update data in database File in PHP

 <?php

include("header.php");

include("dbconn.php");

?>

<?php

if (isset($_GET["id"])) {

    $id = $_GET["id"];

    $sql = "SELECT * FROM students WHERE id = '$id'";

    $result = $conn->query($sql);

    if ($result->num_rows > 0) {

        $row = $result->fetch_assoc();

        print_r($row);

    } else {

        echo "No Record Found<br>";

    }

    $conn->close();

}

?>

<?php

if (isset($_POST["update_students"])) {

    if (isset($_GET["id_new"])) {

        $idnew = $_GET["id_new"];

    }

    $fname = $_POST["f_name"];

    $lname = $_POST["l_name"];

    $age = $_POST["age"];

    $sql2 = "UPDATE students SET first_name='$fname', last_name='$lname', age='$age' WHERE id='$idnew'";

    if ($conn->query($sql2) == TRUE) {

        header("location:index.php?update_msg=YOU HAVE SUCCESSFULLY UPDATED THE DATA");

    } else {

        die("Query Failed : " . $conn->error);

        echo "Error : " . $sql2 . "<br>" . $conn->error;

    }

    $conn->close();

}

?>

<form action="update_page_1.php?id_new=<?php echo $id; ?>" method="post">

    <div class="form-group">

        <label for="f_name">First Name</label>

        <input type="text" id="f_name" name="f_name" class="form-control" value="<?php echo $row["first_name"]; ?>">

    </div>

    <div class="form-group">

        <label for="l_name">Last Name</label>

        <input type="text" id="l_name" name="l_name" class="form-control" value="<?php echo $row["last_name"]; ?>">

    </div>

    <div class="form-group">

        <label for="age">Age</label>

        <input type="text" id="age" name="age" class="form-control" value="<?php echo $row["age"]; ?>">

    </div>

    <input type="submit" class="btn btn-success" name="update_students" value="UPDATE">

</form>

<?php include("footer.php"); ?>