Users can choose an item from a category, and update their choices if they choose to do so. I am displaying the items using the select element. The items are displayed via php to keep the html code succinct.   
     <div class="form-group">   
        <label for="category">category</label>
         <select name="category" id="">               
<?php
    getAllCategories();
?>          
         </select>
     </div>
getAllCategories is a function stored in a separate file.
function getAllCategories() {
global $connection;
$categoryQuery = "SELECT * FROM categories";
$runCategoryQuery = mysqli_query($connection, $categoryQuery);
checkQuery($runCategoryQuery);                               
$row = mysqli_fetch_assoc($runCategoryQuery);
while ($row) {
    $categoryName = $row['category_name'];  
    $categoryId = $row['category_id']; 
    echo "<option value='' name='category_name'> $categoryName </option>";
     $row = mysqli_fetch_assoc($runCategoryQuery);   
   }
}
I am able to display this data correctly. Now if the user wants to change the category, he can click the category section and choose any from the drop-down list. Once he clicks the submit button, the POST request runs.
problem: I am unable to get the updated choice of the user, as I get the following message:
notice: Undefined index: category_name in /Applications/XAMPP/xamppfiles/htdocs/demo/cms/admin/includes/edit_post.php on line 8
The code on that line is :    $postCategory = $_POST['category_name'];
category_name is defined in the getAllCategories function. 
What am I doing wrong?
I've already seen this question, but as I mentioned above, I know what the error means. I have no problem retrieving values from a form or a file. I am having problems with retrieving values from the select element.
 
     
    