GET elements in the url are missing after after submitting in POST format

here is my code

if(isset($_GET['id']) && $_GET['id']!=''){

$id= get_safe_value($con, $_GET['id']);
$res=mysqli_query($con, "select * from `categories` where id='$id'");
$row=mysqli_fetch_assoc($res);
$categories=$row['categories'];
}


if(isset($_POST['submit'])){

$categories= get_safe_value($con, $_POST['categories']);
if(isset($_GET['id']) && $_GET['id']!=''){
    mysqli_query($con, "UPDATE `categories` SET `categories`='$categories' WHERE `categories`.`id`=$id");
}else{
    mysqli_query($con,"INSERT INTO `categories` ( `categories`,`status` ) VALUES ('$categories', '1')");
}
echo "<script>window.location.href='categories.php'</script>";
die();
}

to edit my categories, i sent id to the url like this

echo "<a href='manage-categories.php?id=".$row['id']."'>Edit </a>"; 

but after form submission, that second if block unable to catch the id. Any solution?

this is my form

     <form method="POST" action="manage-categories.php">
      <div class="card">
          <div class="card-header">
           <h4 class="card-title">Insert Categories</h4>
          </div>
          <div class="card-block">
          <div class="card-body">

            <h5 class="mt-2">Category Name</h5>
                  <fieldset class="form-group">
             <input type="text" class="form-control" id="placeholderInput" value="<?php echo $categories; ?>" name="categories" placeholder="Enter category name">
                  </fieldset>
                  <div class="form-group">
                        <input class="btn btn-primary btn-min-width mr-1 mb-1" name="submit" type="submit" value="Insert">
                    </div>
                    <div class="field_error">
                    <?php
                    echo "<p>";
                    echo $msg;
                    echo "</p>";
                    ?></div>
              </div>
          </div>
      </div>
  </div>

This form is used for both updation and insert