<?php
if (isset($_POST[1]) && isset($_POST[2]) && isset($_POST[3]) && isset($_POST[4])) {
$user=$_POST[1];
$eMail=$_POST[2];
$Passw=$_POST[3];

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

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

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

$sql = "SELECT eMail FROM dero WHEN eMail = $eMail AND user = $user AND Password = $Passw";
$result = $conn->query($sql);

if ($result->num_rows > 0) {

      echo 'a';

    }  else {

      echo "b";
    }
}
?>