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);