I am making feedback page, but it is producing some errors. i don't understand what is wrong. Please help me figure out where i am doing wrong. when i submit the form the errors goes away and produce right result,but when i refresh the page it produces same errors again
<?php
$name = $_POST['name'];       // error undefined index name
$suggestion = $_POST['suggest'];  // error undefined index suggest
$opinion = $_POST['opinion']; // error undefined index opinion
$submit = $_POST['submit']; // error undefined index submit
if(isset($submit)){
    $sql= mysqli_query($con,"insert into feedback (name,suggestion,opinion) values ('$name','$suggestion','$opinion')");
    if($sql==true){?>
        <div class="alert alert-info">
       Thankyou for your suggestions. we will notify the admins.
</div>
<?php
    }
}
?>
<body>
<div class="feedback">
<form action="feedback.php" method="post"> 
<h1> Help us improve our website</h1>
<h2>Please drop your suggestion below</h2>
  <div class="form-group">
    <label >Your Name:</label>
    <input type="text" class="form-control" name="name" required>
  </div>
  <div class="form-group">
    <label for="comment">Your suggestion</label>
  <textarea class="form-control" name="suggest" rows="5" id="comment" required ></textarea>
  </div>
    <p>Were you satisfy with this website?</p>
   <label class="checkbox-inline">
      <input type="checkbox" name="opinion"value="1"> yes
    </label>
    <label class="checkbox-inline">
      <input type="checkbox" name="opinion" value="2"> no
    </label>
    <label class="checkbox-inline">
      <input type="checkbox" name="opinion" value="3"> may be
    </label>
    <br /><br />
  <button type="submit" name="submit" class="btn btn-primary">Submit</button>
</form>
</div>
</body>
 
     
     
    