applying a css class to every instance of an echoed php variable
<?php // rclogin.php
    include_once 'rcheader.php';
    echo "<h3>Member Log in</h3>";
    $error = $user = $pass = "";

    if (isset($_POST['user']))
    {
        $user = sanitizeString($_POST['user']);
        $pass = sanitizeString($_POST['pass']);

        if ($user == "" || $pass == "")
        {
            $error = "Not all fields were entered<br />";
        }
        else
        {
            $query = "SELECT user,pass FROM rcmembers
                      WHERE user='$user' AND pass='$pass'";

            if (mysql_num_rows(queryMysql($query)) == 0)
            {
                $error = "Username/Password invalid<br />";
            }

$error = 'Not all fields were entered';

if ( $error ) {
    echo '<span class="error">' . $error . '</span><br/>';
}

class Error
{
    ...

    public function __toString()
    {
        ...
        $output = '<span class="error">' . $output . '</span>';
        return $output;
    }
}

$error = new Error;