Can't get Customer Account Data API working in PHP - php

Question

I'm trying to get Intuit CAD API to work from within a PHP application. I've read the docs using the recommended "SDK" (well, more like a demo app), search the web and tried multiple advice scattered here and there. Here's what I came up with:
// I also use log file for curl - which doesn't add any info
$curl_logfile = fopen('/tmp/curl_debug', 'w+');
$ch = curl_init();
$options = array();
$options[CURLOPT_VERBOSE] = true;
$options[CURLOPT_RETURNTRANSFER] = true;
$options[CURLOPT_TIMEOUT] = 360;
$options[CURLOPT_CERTINFO] = false;
$options[CURLOPT_SSL_VERIFYPEER] = false;
$options[CURLINFO_HEADER_OUT] = true;
$options[CURLOPT_STDERR] = $curl_logfile;
$options[CURLOPT_URL] = $signed_request['signed_url'];
curl_setopt_array($ch, $options);
$raw = curl_exec($ch);
if ($error_num = curl_errno($ch)) {
$error_desc = curl_error($ch);
}
// debug - get all info about the request just issues
$all_data = curl_getinfo($ch);
The code above return 400 as the return code (viewable using curl_getinfo()) and empty string in $raw.
How can I make my PHP app talk to Intuit CAD API?
Also, on the same note - some API method require transferring parameters, so it seem, as part of the path. For example: getInstitutionDetails (v1/institutions/INST-ID). Can I put those variables as parameters? Otherwise, I need to craft each request non-generically, according to the specific API method used :-(
Thanks!