PHP - How to return an error message to a register page

i'm having issues in returning an error message into a register form: I have 2 files (register.php and process_register.php), Pratically into register.php ther's the form from which i pass the request thought POST, while in process_login is executed a function (register($conn,$username,$password))

Here are the code of the files

process_register.php

<?php
if((include 'db_connect.php')==TRUE){echo"OKDB";}
if((include 'functions.php')==TRUE){echo"OKFUNCTIONS";}

sec_session_start();
if(login_check($conn) == true) //Already Logged In!
header('Location: ./index.php');

 else{
if(isset($_POST['username'], $_POST['password'])) { 
    //Got username and pswd
   $username=$_POST['username'];
   $password = $_POST['password']; 

    $registerresult=register($conn,$username,$password);

    if($registerresult==TRUE) //Registration success!
    header('location: login.php');
    else
    header('location: register.php');

    }else { 
   // Not here with a post request.
   echo 'Invalid Request';
                               }
                           }

?>

register($conn,$username,$password)

function register($conn,$username,$password){

    //ifusernamealreadyused

    $alreadyregister="SELECT * FROM User WHERE username='$username';";
    $query_alreadyregister=mysqli_query($conn,$alreadyregister);
    $alreadyregister_rows=mysqli_num_rows($query_alreadyregister);

    if($alreadyregister_rows!=0) //UsernameAlreadyUsed
        return false; 
    else{ //UsernameNotAlreadyUsed

            //RegistrationOK!
            $registration="INSERT INTO User(username,password) VALUES('$username',sha2('$password',512));";
            $query_registration=mysqli_query($conn,$registration);
        return true;
        }//UsernameNotAlreadyUsed

} //Register

My question is: How can i show a message into the form if the username is not 'correct?(returns false) Cheers in advance!