mysql_insert_id(); not returning value after successful row insert

I swear I have poured and poured over every other similar question on this site and others... but I think I'm just missing something. Hopefully someone can point out a silly mistake that my brain is hiding from me. :)

My script inserts values from a form into a table called "notes"

At that point it creates two entries in a table called "relationships" through a function called newRelationship.

The value of the variable "$note_id" is populated via my mysql_insert_id(); and passed into the above function.

When the code is executed, the entry is successfully added to "notes" and the "id" column is given a proper value with auto_increment.

Next, two entries are added to the "relationship" table (as they should).

HOWEVER, my mysql_insert_id() keeps kicking out a value of "0" instead of the new row id.

For the life of me I cant figure out what I'm doing wrong. I even tried creating a new table from scratch with the same results. The kicker is that I use pretty much identical code in other files of my project with no problems. Anyone see what I'm doing wrong?

the code in question

    if ($user->is_loaded())
    {

    if($_POST['project_id']) {
    $project_id = $_post['project_id'];
    $long_note = $_POST['long_note'];
    $created_by = $_POST['created_by'];
    $note_sql="INSERT INTO notes (`long_note`, `added`, `created_by`) VALUES ('$long_note', '$timenow', '$created_by')";
    if (!mysql_query($note_sql,$con))
    {
    die('Error: ' . mysql_error($note_sql));
    }
    else {
    echo "note created Creating relationship ";
    $note_id = mysql_insert_id();

    echo $note_id;
    newRelationship($project_id, "project", $note_id, "note");
    newRelationship($client_id, "client", $note_id, "note");
    echo "note added successfuly";

    }

And my function

function newRelationship($parent_id, $parent_type, $child_id, $child_type)
{

global $sql, $con, $timenow;

$relationship_sql="INSERT INTO `relationships` (`parent_id`, `parent_type`, `child_id`, `child_type`) VALUES ('$parent_id', '$parent_type', '$child_id', '$child_type');";
    if (!mysql_query($relationship_sql,$con))
    {
     die('Error: ' . mysql_error($relationship_sql));
    }
    else {
    echo $parent_type." ".$parent_id." realationship with ".$child_type." ".$child_id." successful ";
}

}

Per @jack's suggestion here is the sql of my notes table

CREATE TABLE `notes` (
  `id` int(10) unsigned zerofill NOT NULL AUTO_INCREMENT,
  `contact_id` int(10) NOT NULL,
  `client_id` int(10) NOT NULL,
  `status` text NOT NULL,
  `long_note` text NOT NULL,
  `added` int(11) NOT NULL,
  `modified` int(11) NOT NULL,
  `edit_by` int(10) NOT NULL, 
  `short_note` text NOT NULL,
  `created_by` int(10) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=295 DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC

Per the documentation:

The ID generated for an AUTO_INCREMENT column by the previous query on success, 0 if the previous query does not generate an AUTO_INCREMENT value, or FALSE if no MySQL connection was established.

The documentation states that it can only return 0 if the query last executed does not generate an AUTO_INCREMENT value, which should mean that your PRIMARY KEY column in notes is not properly setup with auto_increment. I would recommend double-checking that the PRIMARY KEY column in notes is in-fact setup with auto_increment (never hurts to check again!).

Viewing your sample code, you do call mysql_insert_id() immediately after insertion, so there shouldn't be any conflicting queries in between to skew the result.

The only thing I see that may cause an issue is that you're passing the MySQL resource to mysql_query(), but not to mysql_insert_id():

if (!mysql_query($note_sql,$con))
...
$note_id = mysql_insert_id();

There may be a conflict in the resources due to this. Try updating your call to:

$note_id = mysql_insert_id($con);