SQL query with PHP

<?php

function test_input($data) {

    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}

$name = "";
$username = "";
$email = "";
$password = "";

$servername = "localhost";
$username = "";
$password = "";
$dbname = "";

$conn = new mysqli($servername, $username, $password, $dbname);

if($conn->connect_error) {
  die("connection failed: " .$conn->connect_error);
}

if($_SERVER['REQUEST_METHOD'] == "POST") {

    if(empty($_POST["name"]))  {
       $nameErr = "Name is required";
    } else {
       $name = test_input($_POST["name"]);

         if (!preg_match("/^[a-zA-Z ]*$/","$name")) {
        $nameErr = "Only letters and white space allowed for name";
      }
    }

    if(empty($_POST["username"])) {
        $usernameErr = "username is required";
    } else {
    $username = test_input($_POST["username"]);

  }
    $sql = "SELECT * FROM webusers WHERE username = '$_POST['username']'";
    $result = $conn->query($sql);
    if ($result->num_rows > 0) {
      while($row = $result->fetch_assoc()) {
        $usernameErr = "username already exist in our Database"
      }
    }

    if(empty($_POST["email"])) {
    $emailErr = "Email is required";
    } else {
       $email = test_input($_POST["email"]);

       if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
           $emailErr = "Invalid email";
         }
    }
    $sql = "SELECT * FROM webusers WHERE email = '$_POST['email']'";
    $result = $conn->query($sql);
    if ($result->num_rows > 0) {
      while($row = $result->fetch_assoc()) {
        $emailErr = "email already exist in our Database"
      }
    }

    if(empty($_POST["password"])) {
        $passwordErr = "password is required";
    } else {
    $password = test_input($_POST["password"]);

    }



    $stmt = $conn->prepare("INSERT INTO webusers (name, username, email, password) VALUES (?,?,?,?)");

    $stmt->bind_param("ssss", $name, $username, $email, $password);

    $name = $_POST['name'];
    $username = $_POST['username'];
    $email = $_POST['email'];
    $password = $_POST['password'];
    $stmt->execute();

    $report =  "New records created succesfully";

    $stmt->close();
    $conn->close();



    }
?>