p_login.php

<?php
session_start();
require_once("db_connection.php");

if ($_POST) {
    $username = isset($_POST["username"]) ? $_POST["username"] : "";
    $password = isset($_POST["password"]) ? $_POST["password"] : "";
    $username = mysqli_real_escape_string($conn, $username);
    $password = mysqli_real_escape_string($conn, $password);
    if ($username && $password) {
        $query  = "SELECT * ";
        $query .= "FROM useraccount ";
        $query .= "WHERE username = '{$username}' AND ";
        $query .= "     password = '{$password}' ";
        $query .= "LIMIT 1";
        $user_set = mysqli_query($conn, $query);
        if (mysqli_num_rows($user_set) && !mysqli_errno($conn)) {
            $user = mysqli_fetch_assoc($user_set);
            $_SESSION["user_id"] = $user["id"];
            header("Location: admin.php");
            exit;
        } else {
            die("Query failed. " . mysqli_error($conn) . " (" . mysqli_errno($conn) . ")");
        }
        mysqli_free_result($user_set);
    } else {
        # username and password are required fields
        # tell it to user
    }
}
mysqli_close($conn);