unable to find function call_user_func() in laravel

i am using an smpp library from https://github.com/onlinecity/php-smpp/tree/windows-compatible on windows to send message via smpp but I encounter a problem in smppclient.class I am told that

call_user_func(): Argument #1 ($callback) must be a valid callback, function "printDebug" not found or invalid function name

so I need help to be able to implement this function for sending messages.

here is my send message controller

<?php
namespace App\Http\Controllers;
use Exception;
use GsmEncoder;
use SMPP;
use SmppAddress;
use SmppClient;
use TSocket;

$GLOBALS['SMPP_ROOT'] = dirname(__FILE__); // assumes this file is in the root
require_once $GLOBALS['SMPP_ROOT'].'/protocol/smppclient.class.php';
require_once $GLOBALS['SMPP_ROOT'].'/protocol/gsmencoder.class.php';
require_once $GLOBALS['SMPP_ROOT'].'/transport/tsocket.class.php';

class apiSmsSmppController extends Controller{
    
    public function sendSmsSmpp($message,$tel){
        // Simple debug callback
        function printDebug($str) {
            echo date('Ymd H:i:s ').$str."\r\n";
        }
        try {
            // Construct transport and client, customize settings
            $transport = new TSocket('xxx.xxx.xx.xx',2775,false,'printDebug'); // hostname/ip (ie. localhost) and port (ie. 2775)
            $transport->setRecvTimeout(10000);
            $transport->setSendTimeout(10000);
            $smpp = new SmppClient($transport,'printDebug');
            
            // Activate debug of server interaction
            $smpp->debug = true;        // binary hex-output
            $transport->setDebug(true); // also get TSocket debug
            
            // Open the connection
            $transport->open();
            $smpp->bindTransmitter("xxxxx","xxxx");
            
            // Optional: If you get errors during sendSMS, try this. Needed for ie. opensmpp.logica.com based servers.
            //SmppClient::$sms_null_terminate_octetstrings = false;
            
            // Optional: If your provider supports it, you can let them do CSMS (concatenated SMS) 
            //SmppClient::$sms_use_msg_payload_for_csms = true;
            
            // Prepare message
            $message = 'H€llo world';
            $encodedMessage = GsmEncoder::utf8_to_gsm0338($message);
            $from = new SmppAddress(GsmEncoder::utf8_to_gsm0338('SMPP Tést'),SMPP::TON_ALPHANUMERIC);
            $to = new SmppAddress(22670530377,SMPP::TON_INTERNATIONAL,SMPP::NPI_E164);
            
            // Send
            $smpp->sendSMS($from,$to,$encodedMessage);
            
            // Close connection
            $smpp->close();
            
        } catch (Exception $e) {
            // Try to unbind
            try {
                $smpp->close();
            } catch (Exception $ue) {
                // if that fails just close the transport
                printDebug("Failed to unbind; '".$ue->getMessage()."' closing transport");
                if ($transport->isOpen()) $transport->close();
            }
            
            // Rethrow exception, now we are unbound or transport is closed
            throw $e; 
        }
    }
}

?>

here is the function call in smppclient.class

if($this->debug) 
    call_user_func($this->debugHandler, "Received sms:\n".print_r($sms,true));

if($this->debug) {
            call_user_func($this->debugHandler, "Send PDU         : $length bytes");
    call_user_func($this->debugHandler, ' '.chunk_split(bin2hex($header.$pdu->body),2," "));
    call_user_func($this->debugHandler, ' command_id      : 0x'.dechex($pdu->id));
    call_user_func($this->debugHandler, ' sequence number : '.$pdu->sequence);
}

source https://stackoverflow.com/questions/76557377/unable-to-find-function-call-user-func-in-laravel