getting javascript value into php variable

[Mythri]

I want to get the value into php variable from javascript
if i do like this

<script>
var res = 50;
</script>

<?php echo "<script>document.write(res);</script>"; ?>

it works.
but,
i want to do like this

var res=0;
function add()
{
res =8;
}
</script>

<?php echo "<script>document.write(res);</script>"; ?>

it wont work. how can i do this?

[Dormilich]

Generally speaking, if you want to push a value from JavaScript into PHP you need to make a request. Reason being that PHP is a server-side language and JavaScript a client-side language (aside from node).

[Mythri]

how can i do it? please suggest

[Dormilich]

That depends on what you want to accomplish.

[Mythri]

I am getting values from mqtt server and using websockets for this.

My script is like this :

 <script type = "text/javascript">
var connected_flag=0
var mqtt;
var reconnectTimeout = 2000;
var host="localhost";
var port=9001;
var sub_topic="house/#";
function onConnectionLost(){
console.log("connection lost");
document.getElementById("status").innerHTML = "Connection Lost";
document.getElementById("messages").innerHTML ="Connection Lost";
connected_flag=0;
}
function onFailure(message) {
console.log("Failed");
document.getElementById("messages").innerHTML = "Connection Failed- Retrying";
setTimeout(MQTTconnect, reconnectTimeout);
}
var ot ='';
function onMessageArrived(r_message){
out_msg="Message received "+r_message.payloadString+"<br>";
out_msg=out_msg+"Message received Topic "+r_message.destinationName;
//console.log("Message received ",r_message.payloadString);
console.log(out_msg);
document.getElementById("messages").innerHTML =out_msg;
var topic=r_message.destinationName;
if(topic=="house/outside-light")
{
document.getElementById("outside-light").innerHTML =r_message.payloadString;
}
if(topic=="house/outside-temperature")
{
ot = r_message.payloadString; // here i am assigning the value to a variable

//document.getElementById("outside-temp").innerHTML =r_message.payloadString;
}
}
function onConnected(recon,url){
console.log(" in onConnected " +reconn);
}
function onConnect() {
// Once a connection has been made, make a subscription and send a message.
document.getElementById("messages").innerHTML ="Connected to "+host +"on port "+port;
connected_flag=1
document.getElementById("status").innerHTML = "Connected";
console.log("on Connect "+connected_flag);
mqtt.subscribe(sub_topic);
}

function MQTTconnect() {

console.log("connecting to "+ host +" "+ port);
var x=Math.floor(Math.random() * 10000);
var cname="controlform-"+x;
mqtt = new Paho.MQTT.Client(host,port,cname);
//document.write("connecting to "+ host);
var options = {
timeout: 3,
onSuccess: onConnect,
onFailure: onFailure,

};

mqtt.onConnectionLost = onConnectionLost;
mqtt.onMessageArrived = onMessageArrived;
//mqtt.onConnected = onConnected;

mqtt.connect(options);
return false;


}
function sub_topics(){
document.getElementById("messages").innerHTML ="";
if (connected_flag==0){
out_msg="<b>Not Connected so can't subscribe</b>"
console.log(out_msg);
document.getElementById("messages").innerHTML = out_msg;
return false;
}
var stopic= document.forms["subs"]["Stopic"].value;
console.log("Subscribing to topic ="+stopic);
mqtt.subscribe(stopic);
return false;
}
function send_message(msg,topic){
if (connected_flag==0){
out_msg="<b>Not Connected so can't send</b>"
console.log(out_msg);
document.getElementById("messages").innerHTML = out_msg;
return false;
}
var value=msg.value;
console.log("value= "+value);
console.log("topic= "+topic);
message = new Paho.MQTT.Message(value);
message.destinationName = "house/"+topic;

mqtt.send(message);
return false;
}


</script>

And inside function onMessageArrived(r_message), i am assigning a value to a variable and if i try to display that value inside php like this

<?php $foo="<script>document.write(ot)</script>";
echo $foo; ?>

it wont work.
i want that value to be stored in a variable.

[VIPStephan]

I’m not a PHP guy but for me the question that comes up is: what are you trying to do with that variable in PHP?
What you currently have appears to me like a hacky way to do things (document.write() should be avoided at all costs); there must be a way that is more in line with the provided API.

[Dormilich]

It looks to me like you want to display this message on the page. To do that you don't need PHP, you can do that with JS.

[Mythri]

Dormilich : yes, if it is only displaying as it comes, i dont need to store it in a variable and display. But i get some json message, in that i should display only few fields for that i have to store the complete json message in a variable and the process it/display the required fields.

[Dormilich]

But i get some json message, in that i should display only few fields for that i have to store the complete json message in a variable and the process it/display the required fields.

JavaScript is pretty good at processing JSON (it's called JavaScript Object Notation for a reason...), no need to use PHP for that.

But if you really want to use PHP, you need to send a request to PHP, let PHP process that request and send you a response, which you then have to process in JS and then display. You'd do pretty much the same that you would do in JS, only with a round trip to the server.

[Mythri]

Sorry, due to Corona related health issues couldn't reply in the forum.
@Dormilich : can you please share if any example script/tutorial (php) for this? Thanks.