PostgreSQL/PHP: transactions: how-to abstract out?

I posted this on the PHP-general list since it is more of a programming
techniques question that a PostgreSQL question but I thought I would ask
it here also since it has to do with abstracting out the DB layer and I
get the feeling more people on this list have faced this kind of issue
before and have thought about it the issues more in depth.

The problem:

I tough I had abstracted out the SQL querying part of my code out, just
to find out today that it doesn't work when it comes to transactions. I
had come up with this code:

function sql_query($sql) {
   $conn = pg_connect("dbname=JC user=postgres");
   $res  = pg_exec($conn, $sql);
   if (!$res) {
     echo "CONNECTION: could not execute query ($sql)";
     die;
   }
   else return $res;
}

I had transactions in my code implemented like this:

$sql = "BEGIN";
sql_query($sql);
[some sql that should be in a transaction ...]
$sql = "COMMIT";
sql_query($sql);

This doesn't work. Now that I look at my code I can clearly see why. All
sql queries are executed using a new Postgres connection, hence the use
of BEGIN/COMMIT as I was using them have no effect.

Can someone recommend a way to abstract out my DB layer while still
being able to use transactions? (persistent connections are not an
option since there are too many inherent dangers).

I was thinking of using the same function but if the incoming query
contained the word BEGIN, saving that and all future queries in a
session var and when the COMMIT comes in executing all the saved queries
  as one (i.e. "BEGIN;[....];COMMIT"). One drawback is that all queries
will be written out to disk (as session vars) and cause a performance hit.

I was also thinking about maybe the $sql a GLOBAL or first building up
my query as as long string ("BEGIN;[....];COMMIT") and *then* sending it
to my sql_query() function.

The last two seem easier to implement, safer, and more efficient but
they don't seem "elegant" because I haven't really abstracted out the
fact that I want a transaction. Whenever I write an SQL query I have to
think "does this need to be in a transaction" and then use a different
coding technique depending on the answer. And if the future something
that didn't need to be in a transaction now needs to be in a transaction
I have to revisit my code and change the code.

I'm sure someone out there must have thought about this and come up with
an elegant solution and way of abstracting out the DB layer from PHP.

Can anyone share their solution with me or give me some pointers to
reference material?

Thanks,

Jc