HTTP requests while using HTTPS

The url is set by the PHP_SNIPPETS_URL constant, which is predicated on the WP_PLUGIN_URL constant (set by WordPress). In other words, if WP returns an https url for its WP_PLUGIN_URL, then PHP_SNIPPETS_URL will follow suit. If you’re using a plugin to “force” SSL connections, then I suspect it’s too late in the event chain — instead the SSL connection should be forced at the web-server application layer (e.g. in your .htaccess file).

This represents a flagrant flaw in WordPress in my mind: WP is the only CMS application I know of that does not rely on configurable values for its path and URL settings and instead relies on some awkward workarounds to read the values. It makes migrating sites extremely difficult, and in your case, it causes some glitches like this.