选择输入类型=文本值作为列名

php

  $searchby =mysqli_real_escape_string($conn,$_GET['searchby']);
  $searchx =mysqli_real_escape_string($conn,$_GET['searchx']);
  $order=mysqli_real_escape_string($conn,$_GET['order']);
  $fetchquery9 = "SELECT * FROM reservation WHERE '$searchby' = '$searchx' ORDER BY '$order'"; //sql query
  $fetch9 = mysqli_query($conn, $fetchquery9);

ajax

  function sortdata(){
  var searchby = $('#searchby').val();
  var searchx = $('#searchx').val();
  var order = $('#order').val();

  $.ajax({
  type: "POST",
  url: "getdatasort.php?searchby="+searchby+"&searchx="+searchx+"&order="+order
}).done(function( data ) {
  $('#viewdata').html('');
  $('#viewdata').html(data);
 });
 }//passing value using ajax

html

<div class="row">
<div class="col-md-6">
</div>
<div class="col-md-2">
<select type="text" class="form-control" id="searchby" name="searchby"     placeholder="searchby" onChange="sortdata();">
<option value="reservationno">Reservation No.</option>
<option value="name">Full Name</option>
<option value="number">Contact Number</option>
<option value="email">Email</option>
<option value="course_reservation">Course</option>
<option value="class_code_reservation">Class Code</option>
<option value="startdate">Start Date</option>
<option value="enddate">End Date</option>
<option value="desired_date">Desired Date</option>
</select>
</div>
<div class="col-md-2">
<input type="text" class="form-control" id="searchx" name="searchx"             placeholder="Searh here" onChange="sortdata();">
</div>
<div class="col-md-2">
<select type="text" class="form-control" id="order" name="order">
<option value="ASC">ascending</option>
<option value="DSC">descending</option>
</select>
</div>

我只想知道如何在php中的sql查询中将inputtype=text值设置为列名。有3个输入文本，用户可以根据文本值的值查看sql数据。第一个输入包含sql列，第二个输入搜索项，第三个输入顺序（asc或desc）