postgresql 7.3 php transaction problem

Hello

I need a complex transaction that should update some database tables.

The updated tables inside the transaction should be used also to find
information during the transaction process.

BEGIN (I created the transaction)

First iteration
i(0)
I created a record into the 'products_inventory' table (sucess)

Second iteration
i(1)

select pk from products_inventory where (nu_product = 4) and (serial_number
= '9087') and (quantity > 0)
Warning: pg_query(): Query failed: ERROR: current transaction is aborted,
commands ignored until end of transaction block

this query fails on the second iteration (the query is sintactically
correct.

I tried to create a whole batch to lauch the transaction completelly in the
following form

BEGIN;
statement;
statement;
statement;
...
COMMIT;

launching the script from the php page fails, but taking the same script and
using phppgadmin, the same script sucess.

Another question about transactions in postgresql
?Is it possible inside the transaction to see the changes ocurred into the
database ,resulting of the previous statements executed on the same
transaction?

Thanks in advance for any help or explanation

Regards
Enediel