Using ID in a form and have it increment

I've been reading all of the tutorials that I can find, but I can't figure out the most basic thing.

I have a very simple script that I want to use to update a db called sanderan_test.  I have 5 fields. Id, name, short_info, description, and modified.

My form passes the name, short_info, and description over to the php. I understand that and have made it work without having an ID field. Now I want to have it update the ID field. I have the ID field set as an INT in the table and have it set to auto_increment.  How do I get it to increment? Does my php need to tell it to do this or am I trying to fix something that isn't broken?  I'm not able to write any records to the db but not getting any errors.   My php isn't calling the ID or Modified fields, I'm under the assumption these are automatically updated.
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<?php
// Open database connection
$conn = mysql_connect('localhost', 'sanderan_usernam', 'password') or die("Unable to connect, the reason is: " . mysql_error());
mysql_select_db('sanderan_test') or die("Could not select database, the reason is: " . mysql_error());
 
// Pick up the form data and assign it to variables (if it was submitted)
if ($_POST['Status'] == '1') {
$name = $_POST['name'];
$short_info = $_POST['short_info'];
$description = $_POST['description'];
 
 
// Insert data
$query = "INSERT INTO my_new_table ('name', 'short_info', 'description')
  VALUES ('$name', '$short_info', '$description')";
mysql_query($query);
 
 
// Close connection
mysql_close($conn);
 
// Redirect
header("Location: success.html");
}
 
?>