1. Add the current time stamp and assign it to parameter ts to your parameter list. Your time stamp can be in any format.
2. Make sure that you discard all the parameters that don't have assigned values.
3. Sort your parameter list by the parameter names.
4. Build a string : arg0=val0&arg1=val1&arg2=val2 and so on.
5. Append your secret key to the end of the string constructed in step 4.
6. Apply md5 hash to the string constructed in step 5.
7. Assign the hashed value to parameter an_sig.

$arg_assoc_array['ts'] = gmdate("M-d-YTH:i:s");

// Get rid of null parameters. in the assoc array
parse_str(http_build_query($arg_assoc_array,'', '&'), $formatted_arg_assoc_array);

ksort($formatted_arg_assoc_array); // sort by key first
foreach ($formatted_arg_assoc_array as $key =>$val){
    $sig .= $key.'='.$val;
}
$sig .= $secret_key;
$formatted_arg_assoc_array['an_sig'] = md5($sig);

$query = http_build_query($formatted_arg_assoc_array, '', '&');
$url_path = $kt_api_url."/". $version."/".$api_key."/".$api_func."/?$query";