How can wait my Telegram Bot for a incoming message?

How can I let interact my Telegram Bot with Users? E.g.:

User: /buy

Bot: What do you want to buy?

User: Icecream

Bot: You have successfully bought Icecream!

So how can I do this?

switch($message) {
[...]
case "/buy":
    sendMessage($chatID, "What do you want to buy?");
    //Here I need your help :D
    break;
}

Assuming you are using a webhook to receive updates, your php script runs again on every update you are receiving. In this case, you will need to save a certain "status" of the user which you are checking everytime your bot receives a message to indicate what you have to do next. An example would be:

switch($message) {
case "/buy":
    sendMessage($chatID, "What do you want to buy? Icecream, Water or Sprite?");
    $storage[$chatID]['status'] = 'awaitingProductChoice';
    saveStorage();
    break;
}

You need to save $storage[$userId] somehow (saveStorage();). Ideally would be to use a database, if you haven't got one, use file_put_contents('storage.json', json_encode($storage)); or something similar to serialize it. SESSIONS won't work, since Telegram Servers do not send cookies.

Then place some similar code before your switch statement:

$storage = readStorage(); // load from DB or file_get_contents from file
if ($storage[$chatID]['status'] === 'awaitingProductChoice') {
    // return " You have successfully bought Icecream!"
    $storage[$chatID]['product choice'] = $message['text'];
} else if ($storage[$chatID]['status'] === 'other stuff') {
    // other step
}
else switch($message) [...]