Buyur hocam sade bişey yaptım senin için;
<?php
$file = file_get_contents('http://paribu.com/ticker');
$json = json_decode($file);
//var_dump($json);
$bttfiyat=$json->BTT_TL->last;
$gayme=@$_POST["message"];
$varlik=$bttfiyat*$gayme;
?>
<!DOCTYPE html>
<html>
<head>
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<title>BTT Durum</title>
</head>
<body>
<div class="container"> <div class=" text-center mt-5 ">
<h1>Güncel BTT Fiyatı: <?php echo $bttfiyat; ?></h1>
</div>
<div class="row ">
<div class="col-lg-7 mx-auto">
<div class="card mt-2 mx-auto p-4 bg-light">
<div class="card-body bg-light">
<div class="container">
<form id="contact-form" role="form" method="post">
<div class="controls">
<div class="row">
<div class="col-md-12">
<div class="form-group"> <label for="form_message">Mevcut BTT Miktarım *</label> <textarea id="form_message" name="message" class="form-control" placeholder="<?php if(@$_POST["message"]){echo "Toplam Varlığınız :".$varlik." Türk Lirası'dır."; } else {echo "Mevcut BTT Miktarını Yazınız";} ?>" rows="1" required="required" data-error="Lütfen Elinizdeki BTT Miktarını Giriniz"></textarea> </div>
</div>
<div class="col-md-12"> <input type="submit" class="btn btn-success btn-send pt-2 btn-block " value="Hesapla"> </div>
</div>
</div>
</form>
</div>
</div>
</div> <!-- /.8 -->
</div> <!-- /.row-->
</div>
</div>
</body>
</html>