<div class="form-group mb-3">
    <div class="input-group input-group-alternative">
        <div class="input-group-prepend">
            <span class="input-group-text"><i class="fas fa-user"></i></span>
        </div>
        <form action="" method="post">
            <input class="form-control" placeholder="İsim" type="text" name="isim">
            <input class="form-control" placeholder="Soyisim" type="text" name="soyisim">
            <button type="submit" name="tamam">Tamam</button>
        </form>
    </div>
</div>

<?php
if(isset($_POST['tamam'])){
echo $_POST['isim']. " ". $_POST['soyisim'] ;
}
?>