如何在php中存储while循环时使用jquery获取文本框值

There is a probelm in getting a textbox value using jquery it is store in while loop following is my design code

<?php
 while($raw=mysqli_fetch_array($result))
 {
 ?>
    <input type="text" name="comment" id="comment" class="comment"/>
    <input type="button" name="btncmnt" id="btncmnt" class="btncmnt"/>
 <?php
  }
?>

Here is my jquery code

<script type="text/javascript">
    $(document).ready(function(){
        $(".btncmnt").click(function(){                
            var comment=$(this).val(".comment");
            alert(comment);
            //this will be display only first textbox value
        });
    });
</script>

When i click another button of the loop then it will be display first texbox value please give me ans to get current textbox value thank in advance

Firstly note that your PHP loop will be creating invalid HTML as you are generating multiple elements with the same id attribute. As you don't seem to be using it, you should remove it:

<?php while($raw = mysqli_fetch_array($result)) { ?>
  <input type="text" name="comment" class="comment"/>
  <input type="button" name="btncmnt" class="btncmnt"/>
<?php } ?>

Your issue is because your use of val() is incorrect. You need to first use DOM traversal to find the related .comment element, then use the getter of val() to retrieve its value, like this:

$(".btncmnt").click(function(){                
  var comment = $(this).prev(".comment").val();
  console.log(comment); 
});