I have a USD->BTC PHP converter, the user will input a USD amount and click submit.
It works by submitting the form <form action="converter.php method="post">
the user will be redirect to converter.php and see the result to BTC (see the image below)

What i want to do

Instead of redirecting the user to converter.php page, i’d like to POST and display the results into a modal popup (without redirecting)

Currently, echo works from the given user input "Your number is: ". $amount;
but it will not convert this amount to BTC, "Please fill out the form. Go back." is after an if else statement.

index.php

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
<script src="//code.jquery.com/jquery-3.5.1.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>


<!--form-->
<form  id="form" method="post">

<label for="amount">Amount</label>
<input type="text" name="amount" id="amount">
<label for="currency">Currency</label>
<select name="currency" id="currency"><option value="USD">USD</option></select><br>
<button type="button" id="btn" class="btn btn-info" data-toggle="modal" data-target="#myModal">Send Data</button>
</form>



<!--modal-start-->
<div class="modal fade" id="myModal" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">

<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
<h4 class="modal-title" id="myModalLabel" style="text-align:left; float:left; position:absolute;">Title</h4>
</div>

<div class="modal-body">

<div id="bingo"></div> <!--bingo shows response - whatever written in echo of PHP script.-->

</div>
</div>
</div>
</div>
<!--modal-end-->




<!--AJAX Script to receive response from PHP and put into modal-->

<script>
$(document).ready(function(){
$("#btn").click(function(){
var vAmount = $("#amount").val();
$.post("converter.php", //Required URL of the page on server
{ // Data Sending With Request To Server
amount:vAmount ,},
function(response,status){ // Required Callback Function
$("#bingo").html(response); //"response" receives - whatever written in echo of above PHP script.
$("#form")[0].reset();
});
});
});
</script>

converter.php

    <?php
//Echo the given number by user
    if($_POST["amount"])
    {
       $amount = $_POST["amount"];
        // Here, you can also perform some database query operations with above values.
        echo "Your number is: ". $amount;
    }



    // Make sure the user submitted all of the data required
    if(isset($_POST['amount']) && is_numeric($_POST['amount']) && isset($_POST['currency'])) {

    // Use curl to perform the currency conversion using Blockchain.info's currency conversion API
      $ch = curl_init();
      curl_setopt($ch, CURLOPT_URL, "https://blockchain.info/tobtc?currency=" . $_POST['currency'] . "&value=" . $_POST['amount']);
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
      $conversion = curl_exec($ch);

      // Use curl to get current prices and 15 minute averages for all currencies from Blockchain.info's exchange rates API
      curl_setopt($ch, CURLOPT_URL, "https://blockchain.info/ticker");
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
      $prices = json_decode(curl_exec($ch), true);
      curl_close($ch);
    ?>


    <h1>Conversion Results</h1>
    <p><?php echo $_POST['amount']; ?> <?php echo $_POST['currency']; ?> is <?php echo $conversion; ?> BTC.</p>



    <?php
      // Display the pricing chart if we're doing a US Dollar conversion
      if($_POST['currency'] == "USD") { 

      // Use curl to get pricing chart data for the past 60 days
      $ch = curl_init();
      curl_setopt($ch, CURLOPT_URL, "https://blockchain.info/charts/market-price?showDataPoints=true&timespan=60days&show_header=true&daysAverageString=7&scale=0&format= JSON");
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
      $chartdata = json_decode(curl_exec($ch), true);
    ?>
   
    
   <?php } ?>
   <?php } else { ?>
   <p>Please fill out the form. <a href="index.php">Go back.</a></p>
   <?php } ?>

>Solution :

You are not passing currency .. Your if statements are checking for it here –> if($_POST['currency'] == "USD") { And you’re checking for it HERE –> if(isset($_POST['amount']) && is_numeric($_POST['amount']) && isset($_POST['currency'])) {

You need to add the additional parameter:

$.post("converter.php", //Required URL of the page on server
{ // Data Sending With Request To Server
amount:vAmount , currency:'USD'},