Add this snippet to wp-config.php to set up HTTPS/SSL in WordPress behind a proxy

httpd.conf

<VirtualHost *:443>

# ...

RequestHeader set X-Forwarded-Proto "https"
RequestHeader set X-Forwarded-Port "443"

# ...

</VirtualHost>

wp-config.php

<?php
if ($_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https') {
  $_SERVER['HTTPS'] = 'on';
  $_SERVER['REQUEST_SCHEME'] = 'https';
}