How do you upload a file using the API?

function uploadFile($parentId, $fileName) 
    {
        $restVerb = self::RESTVERB_POST;
        $uri = '/' . self::ITEM_CONTACTS . '/' . self::ACTIVITY_FILE . '/';
        
        $boundary = "---------------------".substr(md5(rand(0,32000)), 0, 10); 
           
        $data = "--$boundary\n"; 
        $data .= "Content-Disposition: form-data; name=\"parent\"\n\n" . $parentId . "\n"; 
        $data .= "--$boundary\n"; 
        $data .= "Content-Disposition: form-data; name=\"file\"; filename=\"$fileName\"\n"; 
        $data .= "Content-Type: application/pdf\n"; 
        $data .= "Content-Transfer-Encoding: binary\n\n"; 
        $data .= file_get_contents($fileName)."\n"; 
        $data .= "--$boundary--\n"; 
        
        
        // get the connection handler
        $connection = $this->_connect();

        // Prepare request body
        $request = "$restVerb $uri HTTP/1.1\r\n"
                 . "Host: " . $this->getHost() . "\r\n"
                 // Authorization header
                 . "Authorization: Basic " . $this->getCredentials() . "\r\n"
                 // We inform the server that we're sending data in xml format
                 . "Content-Type: multipart/form-data; boundary=$boundary\r\n"
                 // We inform the server that we are waiting for xml in response
                 . "Accept: application/xml\r\n"
                 . "Content-Length: " . strlen($data) . "\r\n"
                 . "Connection: close\r\n\r\n"
                 . $data;

        // Send request to the host
        fwrite($connection, $request);

        // Get the xml from the response
        $xml = $this->_getXmlResponse($connection);

        // Disconnection
        fclose($connection);

        if ($xml) {
            // Convert it to the SimpleXmlElement
            $xml = simplexml_load_string($xml);
        } else {
            // Something went wrong and we haven't got xml in the response
            throw new Exception('System error while working with Solve360 service');
        }

        return $xml;
    }