在heroku上部署php消息传递应用程序并使用twilio发送消息

I am trying to deploy a sms application which is written in PHP and I am deploying it on Heroku and using Twilio to send messages. When someone sends a message on my Twilio number, this app will send an appropriate reply to that message. I am facing a problem with switch statement in this app. I am new to coding in PHP and I am unable to get this to work. I am pasting my code below. Please help me fix it. Thank you in advance.

<?php
    session_start();
    header("content-type: text/xml");
    switch ($answer) {
        case 'headache' : {
            print('OTC Crocin may help!');
            break;
        case 'stomachache' : {
            print('OTC Tylenol may help!');
            break;
        case 'cough' : {
            print('OTC Robitussin Cough may help!');
            break;
        case 'cold' : {
            print('OTC Aspirin may help!');
            break;  
        case 'vomiting' : {
            print('OTC Pepto-Bismol may help!');
            break;
        case 'headache doctor' : {
            print('Dr. Lorem Ipsum - Here is the address! Call at +1 xxx xxx xxxx');
            break;
        case 'stomachache doctor' : {
            print('Dr. Lorem Ipsum - Here is the address! Call at +1 xxx xxx xxxx');
            break;
        case 'cough doctor' : {
            print('Dr. Lorem Ipsum - Here is the address! Call at +1 xxx xxx xxxx');
            break;
        case 'cold doctor' : {
            print('Dr. Lorem Ipsum - Here is the address! Call at +1 xxx xxx xxxx');
            break;
        case 'vomiting doctor' : {
            print('Dr. Lorem Ipsum - Here is the address! Call at +1 xxx xxx xxxx');
            break;
    }




    $from   = $_POST['From'];
    $answer = $_POST['Body'];
    $reply  = array();


    echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>
";
?>

<Response>
    <Sms>
            <?php
                if(is_array($reply)){
                    foreach($reply as $key => $value){
                        echo $value;
                    }
                }
                else{
                    echo $reply;
                }
            ?>
    </Sms>
</Response>

Twilio developer evangelist here.

You're responding to an incoming SMS message, however you are using the <Sms> TwiML verb. <Sms> is only used for sending an SMS message during a voice call.

Instead, you want to use the <Message> TwiML verb which is for replying to SMS messages. So, the last part of your code should look like:

<Response>
    <Message>
            <?php
                if(is_array($reply)){
                    foreach($reply as $key => $value){
                        echo $value;
                    }
                }
                else{
                    echo $reply;
                }
            ?>
    </Message>
</Response>