LOCKing method in my situation?

Hello PostgreSQL gurus!

I am have question to you.

I am have table 'chall_item' in my database.
In many PHP scripts i am do 'SELECT * FROM chall_item....', but
in one script (join.php) i do
  a) SELECT COUNT(*) FROM chall_item WHERE chall_no = XXX;
  b) if returned number less then 10, then i do INSERT INTO chall_item (chall_no, ...) VALUES (XXX, ...);

But if beetwen a) and b) other instance of 'join.php' do part "a)" - then odd row are added ;(

I can do LOCK TABLE chall_item IN ACCESS EXCLUSIVE MODE,
but IMHO this is not true method in this situation, because i need lock only for 'join.php'
not for all scripts. Help me plz. Which other methods are exist in this situation.

--
Best regards,
 Alexander                          mailto:voodoo@wenet.ru