$ad="ahmet";
$soyad="er";

$birlestir=$ad." ".$soyad;

   <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>
                                            <input class="form-control"
                                                placeholder="İsim" type="text"
                                                name="isim">
                                                <input class="form-control"
                                                placeholder="Soyisim" type="text"
                                                name="soyisim">
                                        </div>
                                    </div>
                                       <? $name =$isim." ".$soyisim; ?>