php function password_verify in not working

can any one help me. I match old and new password using Password_verify function using php. but it is not working.

if (isset($_POST['update_profile'])) {
    $conn = mysqli_connect('localhost','root','','pos');
    $id = $_POST['id'];
    $username = $_POST['username'];
    $email = $_POST['email'];
   $oldpass = $_POST['op'];

    $sql = "SELECT password from users where id = '$id'";

    $result = mysqli_query($conn, $sql) or die('Querry Error');
    if(mysqli_num_rows($result)>0){
        while ($row = mysqli_fetch_assoc($result)) {
       $rowpass = $row['password'];
            if(password_verify($oldpass,$rowpass) ) {
            echo "Password Ok";
            }else{
                echo "Password Not Ok";
            }
        }
    }
}

Hi! can any one help me. I match old and new password using Password_verify function using php. but it is not working.

if (isset($_POST['update_profile'])) {
    $conn = mysqli_connect('localhost','root','','pos');
    $id = $_POST['id'];
    $username = $_POST['username'];
    $email = $_POST['email'];
   $oldpass = $_POST['op'];

    $sql = "SELECT password from users where id = '$id'";

    $result = mysqli_query($conn, $sql) or die('Querry Error');
    if(mysqli_num_rows($result)>0){
        while ($row = mysqli_fetch_assoc($result)) {
       $rowpass = $row['password'];
            if(password_verify($oldpass,$rowpass) ) {
            echo "Password Ok";
            }else{
                echo "Password Not Ok";
            }
        }
    }
}