////////////////////////////////////////////// register.php

$username = filter_var( $_POST[ 'username' ], FILTER_SANITIZE_STRING );
$email =  filter_var( $_POST[ 'email' ], FILTER_SANITIZE_STRING );
$password =  filter_var( $_POST[ 'password' ], FILTER_SANITIZE_STRING );
$password2 =  filter_var( $_POST[ 'password2' ], FILTER_SANITIZE_STRING );
$tos =  filter_var( $_POST[ 'tos' ], FILTER_SANITIZE_STRING );
$session_key = randomstring( 64 );
$ip = getIP();
$registerUser = new register( $username, $email, $password, $password2, $tos, $ip, $session_key, $con );
$registerUser -> test();
if ( $registerUser -> isFailed() ) {
    echo $registerUser -> getErrors();
    $registerUser = null;
} else {
    $registerUser -> register();
    echo 'true';
}

//////////////////////////// register_class.php

<?php
class register{

    private $username;
    private $email;
    private $password;
    private $password2;
    private $tos;
    private $ip;
    private $session_key;
    private $errorMSGS = array( 'error' => array() );
    private $registrationFailed;
    private $con;

    function __construct( $username, $email, $password, $password2, $tos, $ip, $session_key, $con ){
        $this -> username = $username;
        $this -> email = $email;
        $this -> password = $password;
        $this -> password = $password2;
        $this -> tos = $tos;
        $this -> ip = $ip;
        $this -> session_key = $session_key;
        $this -> con = $con;
    }

    public function isFailed() {
        if ( $this -> registrationFailed ) {
            return true;
        } else {
            return false;
        }
    }

    public function getErrors() {
        return json_encode( $this -> errorMSGS );
    }

    public function test() {
        $sql = $this -> con -> prepare( "Select username from forum_users where username = ?" );
        $sql -> bind_param( 's', $this -> username );
        $sql -> execute();
        $sql -> bind_result( $checkusername );
        $sql -> fetch();
        $sql -> close();

        $sql = $this -> con -> prepare( "Select email from forum_users where email = ?" );
        $sql -> bind_param( 's', $this -> email );
        $sql -> execute();
        $sql -> bind_result( $checkemail );
        $sql -> fetch();
        $sql -> close();

        if ( $checkusername != null ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "This username is already taken!");
        }
        if ( $checkemail != null ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "This email is already taken!" );
        }
        if ( !filter_var( $this -> email, FILTER_VALIDATE_EMAIL ) ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "Email is invalid!" );
        }
        if ( $this -> tos == 'false' ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "You have to agree to Terms of Service!" );
        }
        if ( strlen( $this -> username ) < 3 ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "Username must be at least 3 characters long!" );
        }
        if ( strlen( $this -> password ) < 8 ) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "Password must be at least 8 characters long!" );
        }
        if ( $this -> password == $this -> password2) {
            $this -> registrationFailed = true;
            array_push( $this -> errorMSGS[ 'error' ], "Passwords do no match!" );
        }
    }

    public function register() {
        $uuid = randomstring( 32 );
        $salt = sprintf( "$2a$%02d$", 10 ) . randomstring( 55 );
        $encrypedpass = crypt( $this -> password, $salt );
        $rank = 'member';

        $sql = $this -> con -> prepare( "insert into forum_users ( uuid, username, email, password_hash, salt, display_name, rank )  values ( ?, ?, ?, ?, ?, ?, ? )" );
        $sql -> bind_param( 'sssssss', $uuid, $this -> username, $this -> email, $encrypedpass, $salt, $this -> username, $rank );
        $sql -> execute();
        $sql -> close();

        $sql =  $this -> con -> prepare( "insert into forum_sessions ( session_key, ip, userUUID )  values ( ?, ?, ? )" );
        $sql -> bind_param( 'sss', $this -> session_key, $this -> ip, $uuid );
        $sql -> execute();
        $sql -> close();

        session_start(['cookie_lifetime' => 259200]);
        setcookie("forum", $this -> session_key, time() + ( 259200 ), '/');
    }

    private function randomstring( $size ){
        $list = array( 'a', 'b', 'c', 'd', 'e', 'f', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' );
        $string = '';
        for( $x = 0; $x < $size; $x++ ){
          $string = $string . $list[ rand( 0, count( $list ) - 1 ) ];
        }
        return $string;
    }
}
?>