Thread safe way to get auto incremented primary key inserted into another table (so they can be joined later)
$result = mysql_query("SHOW TABLE STATUS LIKE 'table_name'");
$row = mysql_fetch_array($result);
$nextId = $row['Auto_increment'];
mysql_free_result($result);

$sql = mysql_query("INSERT the information you want in the main table")

START TRANSACTION;
INSERT INTO files (file_id, url) VALUES (NULL, 'text.doc');
INSERT INTO grades (file_id, grade) VALUES (LAST_INSERT_ID(), 'some-grade');
COMMIT;