php curl - multipart/mixed issue

I am trying to send curl requests to google batches api, but i get an error (resource(2) of type curl).

Google Batch Documentation: https://developers.google.com/calendar/batch#batch-example-request

Here is my code:

function buildMixedRequest() {
    $CAL_BASE_URL  = 'https://www.googleapis.com/calendar/v3/calendars/';
    $CAL_BATCH_URL = 'https://www.googleapis.com/batch';        

    $ch = curl_init( $CAL_BATCH_URL );

    $events = array();
    $events[] = array( 'calendar_id' => 'primary', 'BookingRef' => '#1', 'StartDate' => '2018-06-6T09:05:00', 'EndDate' => '2018-06-6T11:05:00' );
    $events[] = array( 'calendar_id' => 'primary', 'BookingRef' => '#2', 'StartDate' => '2018-06-7T11:05:00', 'EndDate' => '2018-06-7T11:55:00' );  

    $calendar_id = 'primary';
    $access_token = '{my_token_here}';
    $boundary = 'batch_' . uniqid();
    $data = '';

    //$data .= "--" . $boundary . "\r\n";
    //$data .= 'Content-Type: application/http';

    // Add batches
    foreach ( $events as $event ) {
        $data .= "--" . $boundary . "\r\n"
            . 'PUT ' .  $CAL_BASE_URL . $calendar_id . '/events' . "\r\n"
            . 'Content-Type: application/json' . "\r\n"
            . 'Content-Length: ' . strlen(json_encode($event)) . "\r\n";
        $data .= json_encode($event) . "\r\n\r\n";
    }

    // Headers
    $headers = array(
        "Authorization: Bearer " .  $access_token,
        "Host: www.googleapis.com",
        "Content-Type: multipart/mixed; boundary={$boundary}",
        'Content-Length: ' . strlen(json_encode($events))
    );  

    // Curl Setup
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
    curl_setopt($ch, CURLOPT_POST, true);       
    curl_setopt($ch, CURLINFO_HEADER_OUT, true);
    curl_setopt($ch, CURLOPT_VERBOSE, 1);   
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); 
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);    
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, false);        
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);

    // Execute
    curl_exec($ch);

    // Get INFO
    $info = curl_getinfo($ch, CURLINFO_HEADER_OUT);

    print_r($info);
    print_r($data);

    // Make CURL reponse
    // $response = json_decode(curl_exec($ch));

    var_dump($ch);          

    // Close CURL
    curl_close($ch);        

    return $ch;
}

buildMixedRequest();

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The curl info output is:

POST /batch HTTP/1.1
Host: www.googleapis.com
Accept: */*
Authorization: Bearer {my_token_here}
Content-Type: multipart/mixed; boundary=batch_5b18f5ea54b04
Content-Length: 217

--batch_5b18f5ea54b04
PUT https://www.googleapis.com/calendar/v3/calendars/primary/events
Content-Type: application/json
Content-Length: 107
{"calendar_id":"primary","BookingRef":"#1","StartDate":"2018-06-6T09:05:00","EndDate":"2018-06-6T11:05:00"}

--batch_5b18f5ea54b04
PUT https://www.googleapis.com/calendar/v3/calendars/primary/events
Content-Type: application/json
Content-Length: 107
{"calendar_id":"primary","BookingRef":"#2","StartDate":"2018-06-7T11:05:00","EndDate":"2018-06-7T11:55:00"}

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What i am doing wrong? Can't see where the issue is.