Need response body of HTTP 500 with file_get_contents (PHP)

Using file_get_contents as part of custom SOAP implementation to apply SOAP calls (ALL libraries that we tried would not do SSL + certificate based authentication with SOAP 1.2 correctly). However difficult and barely-documented API often returns 500. There are error details in response body, but file_get_contents doesn’t appear to allow access to it. fopen seems to have the same issue.