/**
 * Send a message to a Threema ID.
 *
 * @param Threema\MsgApi\Connection $connector connector
 * @param string                    $threemaId The id where the message should be send to
 * @param string                    $message   The message to send (max 3500 characters)
 *
 * @throws Exception
 * @return int
 */
function SendMessage(Threema\MsgApi\Connection $connector, $threemaId, $message)
{
    $e2eHelper = new \Threema\MsgApi\Helpers\E2EHelper(KeyHexToBin(MSGAPI_PRIVATE_KEY), $connector);
    $result = $e2eHelper->sendTextMessage($threemaId, $message);
    if (true === $result->isSuccess()) {
        return $result->getMessageId();
    } else {
        throw new Exception($result->getErrorMessage());
    }
}
<?php

use Threema\MsgApi\Connection;
use Threema\MsgApi\ConnectionSettings;
//include_project
require_once 'bootstrap.php';
//define your connection settings
$settings = new ConnectionSettings('*YOUR_GATEWAY_THREEMA_ID', 'YOUR_GATEWAY_THREEMA_ID_SECRET');
//public key store file
//best practice: create a publickeystore
//$publicKeyStore = new Threema\MsgApi\PublicKeyStores\PhpFile('keystore.php');
$publicKeyStore = null;
//create a connection
$connector = new Connection($settings, $publicKeyStore);
$senderPrivateKey = "MY_PUBLIC_KEY_IN_BIN";
$filePath = "/path/to/my/file.pdf";
$e2eHelper = new \Threema\MsgApi\Helpers\E2EHelper($senderPrivateKey, $connector);
$result = $e2eHelper->sendFileMessage("TEST1234", $filePath);
if (true === $result->isSuccess()) {
    echo 'File Message ID: ' . $result->getMessageId() . "\n";
} else {
    echo 'Error: ' . $result->getErrorMessage() . "\n";
}
<?php

use Threema\MsgApi\Connection;
use Threema\MsgApi\ConnectionSettings;
//include_project
require_once 'bootstrap.php';
//define your connection settings
$settings = new ConnectionSettings('*YOUR_GATEWAY_THREEMA_ID', 'YOUR_GATEWAY_THREEMA_ID_SECRET');
//public key store file
//best practice: create a publickeystore
//$publicKeyStore = new Threema\MsgApi\PublicKeyStores\PhpFile('keystore.php');
$publicKeyStore = null;
//create a connection
$connector = new Connection($settings, $publicKeyStore);
$senderPrivateKey = "MY_PUBLIC_KEY_IN_BIN";
$e2eHelper = new \Threema\MsgApi\Helpers\E2EHelper($senderPrivateKey, $connector);
$result = $e2eHelper->sendTextMessage("TEST1234", "This is an end-to-end encrypted message");
if (true === $result->isSuccess()) {
    echo 'Message ID: ' . $result->getMessageId() . "\n";
} else {
    echo 'Error: ' . $result->getErrorMessage() . "\n";
}