WordPress wp-config.php customization for multiple domains

multiple-domains.php

<?php

if (isset($_SERVER['HTTP_HOST'])) {
    // HTTP is still the default scheme for now. 
    $scheme = 'http';
    // If we have detected that the end use is HTTPS, make sure we pass that
    // through here, so <img> tags and the like don't generate mixed-mode
    // content warnings.
    if (isset($_SERVER['HTTP_USER_AGENT_HTTPS']) && $_SERVER['HTTP_USER_AGENT_HTTPS'] == 'ON') {
        $scheme = 'https';
    }
    define('WP_HOME', $scheme . '://' . $_SERVER['HTTP_HOST']);
    define('WP_SITEURL', $scheme . '://' . $_SERVER['HTTP_HOST']);
}