How to query OData API using PHP with a API Key and API Secret? Getting 'No URL set!' curl error

Question

I am new to APIs and I am trying to create an application which would fetch some data from an OData API provided by Cornerstone OnDemand LMS. Its called Reporting API. The Cornerstone OnDemand documentation says, I need to obtain the API Key and API secret and then acquire the session token. It explains the process until generating the API session signature and provides a sample PHP code with it. However I am not sure how to go beyond that after the signature is generated.

Here is the sample PHP code available in Cornerstone OnDemand documentation.

//signature to acquire a session
$apiId = '<insert Api ID>';
$apiSecret = '<Insert API Secret>';

//build the string to sign
//note the order of the entries is important.
//The http headers must be in alphabetical order by key name
$httpMethod = 'POST';
$apiKey = 'x-csod-api-key:'.$apiId;
$httpUrl = '/services/api/sts/session';

date_default_timezone_set('UTC');
$date = 'x-csod-date:'.date('Y-m-d').'T'.date('H:i:s').'.000';
$stringToSign = $httpMethod."\n".$apiKey."\n".$date."\n".$httpUrl;

/* produces the following string:
*  POST\nx-csod-api-key:1lie8ficql9h5\nx-csod-date:2015-09-08T11:27:32.000\n/services/api/sts/session
*/

//Generate the signature
$secretKey = base64_decode($apiSecret);
$signature = base64_encode(hash_hmac('sha512', $stringToSign, $secretKey, true));

I am not sure what to do after the signature is generated. A REST API sample C# code is available on the GitHub repository of Cornerstone OnDemand at https://github.com/csodedge/csod-rest-api-sample-code-POST however I can't find anything that I could use for PHP.

I look forward to some help from the StackOverflow community. I will be very grateful for any help.

EDIT 1: After some research, I figured that I need to form the headers and use Curl to send the request. I have added the curl code, however I am now getting a bad request error. Here is the updated code. Any help is much appreciated.

//signature to acquire a session
    $apiId = '<removed for safety>';
    $apiSecret = '<removed for safety>';

    //build the string to sign
    //note the order of the entries is important.
    //The http headers must be in alphabetical order by key name
    $httpMethod = 'POST';
    $apiKey = 'x-csod-api-key:'.$apiId;
    $httpUrl = 'https://clientdomain.csod.com/services/api/sts/session?userName=clientuserid&alias=jk01';

    date_default_timezone_set('UTC');
    $date = 'x-csod-date:'.date('Y-m-d').'T'.date('H:i:s').'.000';
    $stringToSign = $httpMethod."\n".$apiKey."\n".$date."\n".$httpUrl;

    //Generate the signature
    $secretKey = base64_decode($apiSecret);
    $signature = base64_encode(hash_hmac('sha512', $stringToSign, $secretKey, true));

    $crl = curl_init();

    curl_setopt($crl, CURLOPT_URL, $httpUrl); 
    curl_setopt($crl, CURLOPT_HTTPHEADER, array (
      'x-csod-api-key: '.$apiId,
      'x-csod-date: '.date('Y-m-d').'T'.date('H:i:s').'.000',
      'x-csod-signature: '.$signature
    ));
    curl_setopt($crl, CURLOPT_POST,true);
    $rest = curl_exec($crl);
    if ($rest === false)
    {
        // throw new Exception('Curl error: ' . curl_error($crl));
        print_r('Curl error: ' . curl_error($crl));
    }

    curl_close($crl);
    print_r($rest);

This code is now giving me the following error:

Bad Request Your browser sent a request that this server could not understand. Reference #7.9dde387d.1532191458.b66f123 1

Please help.