Why won't my mysql_insert_id work?

Below is the final step of my form processor. I need to get the job_id (Primary Key, Auto Increment) from the last Insert and pass it into the URL in the Header: location bit.
For some reason I can't figure out why its not doing it.

I've used the mysql_insert_id() before but I'm not getting any returned value.
any help is appreciated
-a

//---------INSERT-----------//
		
// Set up the SQL query and get it ready for input
$sql = "INSERT INTO timecard_data 
			(emp_id, prop_id, unit_id, work_type, job_duration, job_notes, job_charge, job_date)
			VALUES
			('$emp_id', '$prop_id', '$unit_id', '$work_type', '$job_duration', '$job_notes', '$job_charge', '$job_date')";

// make the actual input into the database
mysql_query($sql,$conn) or die(mysql_error());
			mysql_close($conn);
 
 //---------ADD. ACTIONS-----------//
 
 // redirect the user to back to his "dashboard" using the emp_id variable
 $id = mysql_insert_id();
 $url = ("emp-timecard.php?emp_id=" . $emp_id . "&job_id=" . $id);
 header("Location: $url");
		exit();

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