brown adlı üyeden alıntı: mesajı görüntüle
<form action="#" method="POST">
<input class="form-control" placeholder="İsim" type="text" name="isim">
<input class="form-control" placeholder="Soyisim" type="text" name="soyisim">
<input type="submit" name="formSubmit" value="gönder">
</form>
<?php
if(isset($_POST['formSubmit'])){
$name = $_POST['isim']." ".$_POST['soyisim'];    
echo $name;
}
?>
Misafir adlı üyeden alıntı: mesajı görüntüle
<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'] ;
}
?>
Tekrardan merhabalar, yanıtlarınız için teşekkür ederim. Ancak halen aynı hatayı alıyorum. Kodun tamamı şu şekilde;

                    <div class="form-container">
                               <form action="/ajax/register" method="POST">
                                    <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>
                                    <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-envelope"></i></span>
                                            </div>
                                            <input required class="form-control"
                                                placeholder="<?php echo $this->lang->line("enter_email"); ?>"
                                                type="email" name="email">
                                        </div>
                                    </div>
                                           <div class="form-group">
                                        <div class="input-group input-group-alternative">
                                            <div class="input-group-prepend">
                                                <span class="input-group-text"><i class="fas fa-key"></i></span>
                                            </div>
                                            <input required class="form-control"
                                                placeholder="Parola oluşturun"
                                                type="password" name="password">
                                        </div>
                                    </div>
                                    <div class="form-group mb-3">
                                        <div class="input-group input-group-alternative" >
                                            <div class="input-group-prepend"  style="display:none;>
                                                <span class="input-group-text"><i class="fas fa-map"></i></span>
                                            </div>
                                            <input required class="form-control"  style="display:none;
                                                placeholder="<?php echo $this->lang->line("enter_address"); ?>"
                                                type="text" name="address" value="İstanbul/Türkiye">
                                           <div class="input-group-prepend" style"width:50%;"  style="display:none;>
                                                <span class="input-group-text"><i class="fas fa-map-marker"></i></span>
                                            </div>
                                            <input required class="form-control"
                                                placeholder="<?php echo $this->lang->line("enter_city"); ?>"
                                                type="text" name="city" value="İstanbul"  style="display:none;">
                                        </div>                                        </div>
                                    <div class="text-center">
                                        <button type="submit"
                                           class="form-control-submit-button"  name="formSubmit">Kaydol</button>
                                    </div>
                                    <input type="hidden" name="recaptcha_response" id="recaptchaResponse">
                                </form>
                    </div> <!-- end of form container -->
<?php
if(isset($_POST['formSubmit'])){
$name = $_POST['isim']." ".$_POST['soyisim'];
echo $name;
} ?>