some configuration string would be better

This is actually a pretty bad idea.  The autocommit setting 
should be controlled either by making it fixed at on or off 
for the interface, or by the client program itself (e.g., a 
"set autocommit on" function), but not by a global setting, 
because then you'll never be able to reconcile various 
prepackaged PHP applications.  The excessive global-ness 
was the reason that the server-side autocommit setting was 
removed in the first place.

Autocommit considered harmful.

The consumers of database resources within an application server should decide how transactional behaviour works, not the application server itself.

This is analogous to the rule (now, too frequently broken) that MTAs should not mess with message bodies.

What if my app has a multi-request series of action that should succeed or fail as a group?

QUOTE:
What if my app has a multi-request series of action that should succeed
or fail as a group?
UNQUOTE:

Then you need to enclose them in begin;/end; pairs.  Simple enough.

I'm really not Postgresql expert (never used it actually :),
but according to this documentation:

  http://www.postgresql.org/docs/7.4/static/sql-begin.html

"By default (without BEGIN), PostgreSQL executes transactions in "autocommit" mode..."

So if I understood that correctly, to turn it off you just need to begin your queries with 'BEGIN' and end with 'COMMIT' to "disable" autocommit?

Now why would you need yet another php.ini option for this?!

Yes ,

You people are right 
Let me clarify :
1. PostgreSQL Version 7.4 > are by default in autocomit mode 
   i.e the postgresql.conf entry has been removed to control this
2. Try doing this on < 7.4 
   cs=# set autocommit=off;
   SET 
    This means Sucess
   But on 7.4 this fails
3. So whosoever wants to control the Autocomit feature would have to work with BEGIN AND COMMIT block
What does this mean :-
That all those application that where not using BEGIN and COMMIT block with PostgreSQL < 7.4 will easylu port to 7.4
But
Those who were relying on postgresql.conf feature will have 
have to look around and introduce BEGIN and COMMIT block.

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If I am wrong then please take this bug as feature request