Do you see the "/vendor/ringcentral/" folder under your project folder? Or check if that exists where you install SDK into.
                
     
                                    
            Yes. I see vendor/ringcentral and all of the other files.
My script includes (variables below are defined)
require('vendor/autoload.php');
use RingCentralSDKSDK;
$rcsdk = new SDK($RINGCENTRAL_CLIENTID, $RINGCENTRAL_CLIENTSECRET, $RINGCENTRAL_SERVER);
and I actually echoed variables to the screen to make sure each file was included (as I executed from the Linux command line)... It does include SDK.php.
The error specified is on the line where it says "new SDK" ( figured I'd mention it).
Also... I noticed that the autoload_classmap.php file is basically empty. Unsure if this should be the case:
<?php
// autoload_classmap.php @generated by Composer
$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);
return array(
);
                
     
                                    
            I saw this discussion as well. I did the below and it is bool(true).
I even echo debug output from SDK.php and it works... although, it doesn't see the class SDK when I do get_declared_classes().
<?php
require('vendor/autoload.php');
var_dump(file_exists("vendor/autoload.php"));
                
     
                                    
            Hi Ellen, have you solved this issue yet? Looking at your code I can't tell what server you are trying to connect to. I use the more direct approach by loading straight from the Class. Here is my code for both sandbox and production (I control how I want to connect with a DB table entry of sandbox on or off:
if ($ringcentral_sandbox == 1) {
        // Use sandbox Client
        $sdk = new RingCentralSDKSDK($client_id, $client_secret, RingCentralSDKSDK::SERVER_SANDBOX);        
    } else {
        // Use Production Client
        $sdk = new RingCentralSDKSDK($client_id, $client_secret, RingCentralSDKSDK::SERVER_PRODUCTION);
    }  
This removes the need to have the server name typed in correctly. Not sure this is the culprit but it may make your code a little more streamlined.
Basically I'm just replacing the Server parameter with: 
RingCentralSDKSDK::SERVER_SANDBOX
or
RingCentralSDKSDK::SERVER_PRODUCTION