validating user signup

account.php

<?php
    //including the header
    include 'header.php'
?>

<?php
    //checking if all fields in the signup are empty
    $url = "http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
    if (strpos($url, 'error=empty') !== false) {
        echo "Fill out all fields";
    }

    //checking if the username already exist
    if (strpos($url, 'error=username') !== false) {
        echo "User already";
    }

    //session to check if your are logged in
     if (isset($_SESSION['id'])){
        echo $_SESSION['id'];
     } else {
        echo "you are not logged in";
     }
?>