<?php
function login_user($username, $password) {
    $sqlQuery = "SELECT * FROM users WHERE username = ? AND password = ?";
    $statement = $db->prepare($sqlQuery);
    $statment->bindParam(1, $username);
    $statment->bindParam(2, $password);
    $statement->execute();
    $data = $statement->fetch();
    if ($data) {
        return true;
    }
    return false;
}

if (isset($_POST['submit'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    // execute your function
    if (login_user($username, $password)) {
        header("location:admin");
    }
}

header("location:login.php");