login.php

<?php
session_start();
require "db.php";

$email = ($_POST['email']);
$password = ($_POST['password']);

$result = $db->query("SELECT * FROM Users WHERE username='$email'")->fetchAll();

if (count($result) != 0) { // User exists
    $user = $result[0];

    if (password_verify($password, $user['password'])) {
        $_SESSION['email'] = $email;
        $_SESSION['logged_in'] = true;

        $message = "OK";
        $code = 0;
    } else {
        $message = "You have entered wrong password, try again!";
        $code = 1;
    }
} else {
    $message = "User with that email doesn't exist!";
    $code = 2;
}

$arr = array('code' => $code, 'message'=>$message);
header('content-type: application/json');
echo json_encode($arr);
?>