<?php
$con = mysqli_connect('localhost','root','','polo');

function login($email,$pass) {
    // function cannot access variable that is out of function scope, so you would need to use global
    global $con;
    $email = mysqli_real_escape_string($con,$email);
    $pass = mysqli_real_escape_string($con,$pass);
    $query = "SELECT * FROM customer WHERE email='".$email."' AND password='".$pass."'";
    // connection failure
    if (!$result = mysqli_query($con,$query)) {
        echo 'Error :' . mysqli_error($con);
        return false;
    }
    // no matching record
    if (mysqli_num_rows($result) == 0) {
        echo 'Warning: Invalid Login';
        return false;
    }
    return true;
}

if (ISSET($_POST['login'])) {
    $email=$_POST['email'];
    $pass=$_POST['pass'];
    if (login($email,$pass)) {
        echo 'Login Successful';
    }
}
?>