PHP Login System

<?php

   require 'core/init.php';

   if(isset($_POST["submit"])) {

      // Declaration Variable
      $email = $_POST["email"];
      $password = $_POST["password"];

      // Query Retreived Data
      $query = "SELECT * FROM users WHERE email = '$email'";
      $result = mysqli_query($conn, $query);

      // Checking Data Email
      if(mysqli_num_rows($result) > 0) {

         // Declaration Data Users
         $user = mysqli_fetch_assoc($result);

         if($user["password"] === $password) {
            $_SESSION["user"] = $user["email"];
            header("Location: index.php");
         }

      }

      $error = true;

   }

?>

<?php if(isset($error)) : ?>
   <p style="color: red; font-style: italic;">Your email or password invalid</p>
<?php endif; ?>