edit

<html><meta charset="utf-8">
<?php
/**
 * Created by PhpStorm.
 * User: 455
 * Date: 16.12.2018
 * Time: 20:15
 */

if(isset($_GET['id'])){
    $id = $_GET['id'];
    $link = mysqli_connect ("localhost" , "root" , "", 'test');


    if($_POST['update'] == true) {

        $name = $_POST['name'];
        $birthdate = $_POST['birthdate'];
        $experience = $_POST['experience'];

        $sql = "UPDATE users SET name='$name', birth_date='$birthdate', experience='$experience' WHERE id=$id";
        if ($link->query($sql) == TRUE) {
            echo '<meta http-equiv="refresh" content="0;index.php">';
        } else {
            echo "Error updating record: " . $link->error;
        }
    }


    $ath = mysqli_query($link, "select * from users where id=$id;");
    $user = mysqli_fetch_array($ath);
?>

    <form method="POST" action="edit.php?id=<?=$id?>">
        <input type="hidden" name="update" value="true">
        <label>Имя</label><input type="text" name="name" value="<?=$user['name']?>"><br>
        <label>Дата рождения</label><input type="date" name="birthdate" value="<?=$user['birth_date']?>"><br>
        <label>Опыт</label><input type="text" name="experience" value="<?=$user['experience']?>"><br>
        <button type="submit">Сохранить</button><br>
    </form>

<?php
}