The infamous php if hell refactoring

register.php

<?php

function register()
{
    if (empty($_POST)) {
        return register_form();
    }

    if (($msg = validate($_POST)) !== null) {
        $_SESSION['msg'] = $msg;
        return register_form();
    }

    createUser();
    $_SESSION['msg'] = 'You are now registered so please login';
    header('Location: ' . $_SERVER['PHP_SELF']);
    exit();
}

function validate($input)
{
    if (!$input['user_name']) {
        return 'Empty Username';
    }
    if (!$input['user_password_new']) {
        return 'Empty Password';
    }
    if ($input['user_password_new'] !== $input['user_password_repeat']) {
        return 'Password do not match';
    }
    if (strlen($_POST['user_password_']) <= 5) {
        return 'Password must be at least 6 characters';
    }
    if (!preg_match('/^[a-z\d]{2,64}$/i', $input['user_name'])) {
        return 'Username must be only a-z, A-Z, 0-9';
    }
    $user = read_user($_POST['user_name']);
    if (isset($user['user_name'])) {
        return 'Username already exists';
    }
    if (!$input['user_email']) {
        return 'Email cannot be empty';
    }
    if (!strlen($input['user_email']) < 65) {
        return 'Email must be less than 64 characters';
    }
    if (filter_var($input['user_email'], FILTER_VALIDATE_EMAIL)) {
        return 'You must provide a valid email address';
    }
    return null;
}