PostgreSQL locking from PHP scripts

Hello,
I am unable to solve a locking problem. I am using
postgresql database for this project. Here script one
(booking.php) tries to setup a lock for update and ask
user to fill-up the form. Now when the user fill-up
the form and submit it for update, second script
(update.php) update the database and releasing the
locks. This is working for client (say) A and B from
psql prompts but failed from PHP scripts. I have
written following scripts as follows.

booking.php
-----------

...
$db=pg_pconnect($host,$database);
// PostgreSQL database
$sql_str="BEGIN; SELECT * FROM seat WHERE seat_no=1
FOR UPDATE NOWAIT";
...
<FORM method="get" action="update.php">
// collecting user's input
...

update.php
----------
...
$db=pg_pconnect($host,$database);
$sql_str="UPDATE seat SET status='booked' WHERE
seat_no=1; END";
...

// end

My update script failed to do lock the table and
records. How I can fix it? One WARNING is showing by
PostgreSQL, "there is no tranaction in progress".

Thanks for your help in advance.

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