Setting $_SERVER[‘HTTPS’]=’on’ prevents access to wp-admin

Special thanks to user42826.

According to the codex:

If WordPress is hosted behind a reverse proxy that provides SSL, but is hosted itself without SSL, these options will initially send any requests into an infinite redirect loop. To avoid this, you may configure WordPress to recognize the HTTP_X_FORWARDED_PROTO header (assuming you have properly configured the reverse proxy to set that header).

The following actions will solve the problem.

Add this to wp-config.php. (codex reference)

/* SSL Settings */
define('FORCE_SSL_ADMIN', true);

/* Turn HTTPS 'on' if HTTP_X_FORWARDED_PROTO matches 'https' */
if (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false) {
    $_SERVER['HTTPS'] = 'on';
}

Remove this from functions.php as it is unnecessary.

/**
 * Make PHP HTTPS aware via HTTP_X_FORWARDED_PROTO
 */
if(isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
    $_SERVER['HTTPS']='on';
}