<?php

$user = "root";
$pass = "";
$loggedin;
$conn = new PDO('mysql:host=localhost;dbname=login', $user, $pass);

if (!$conn) {
    die("Connection to the database failed");
}

<?php

include '../dbh.php';

$first = $_POST['first'];
$last = $_POST['last'];
$uid = $_POST['uid'];
$pwd = $_POST['pwd'];

if (empty($first)) {
    header("Location: ../signup.php?error=empty");
    exit();
} if (empty($last)) {
    header("Location: ../signup.php?error=empty");
    exit();
} if (empty($uid)) {
    header("Location: ../signup.php?error=empty");
    exit();
} if (empty($pwd)) {
    header("Location: ../signup.php?error=empty");
    exit();
} else {

    $sql = "SELECT uid FROM users WHERE uid='$uid'";
    $result = mysqli_query($conn, $sql);
    $uidcheck = mysqli_num_rows($result);

    if ($uidcheck > 0) {
        header("Location: ../signup.php?error=username");
        exit();
    } else {
        $sql = "INSERT INTO users (first, last, uid, pwd)
        VALUES ('$first', '$last', '$uid', '$pwd')";

        $result = mysqli_query($conn, $sql);

        header("Location: ../index.php");
    }
}