I am a programmer/analyst at MIT and we are requesting this as well. We are currently working on this issue as well. Please let us know if this is on the roadmap and/or if you would accept patches to add this functionality.

Partial functionality exists.

Pre-Test requirements:
create user app identified by "ppa";
create user bob identified by "obo";
grant create session to app;
grant create sesion to bob;
alter user bob connect through app;

PHP Test
<?php

require dirname(__FILE__)."/connect.inc";

$pc = oci_connect("app[bob]", "ppa", $dbase);
$stmt = oci_parse($pc, "select user from dual");
oci_execute($stmt);
var_dump(oci_fetch_array($stmt, OCI_RETURN_NULLS));
$t = oci_close_proxy_session($pc);
echo "Done\n";
?>

result:
array(2) {
  [0]=>
  string(3) "BOB"
  ["USER"]=>
  string(3) "BOB"
}

EDIT to remove oci_close_proxy_session line.

--
Partial functionality exists.

Pre-Test requirements:
create user app identified by "ppa";
create user bob identified by "obo";
grant create session to app;
grant create sesion to bob;
alter user bob connect through app;

PHP Test
<?php

require dirname(__FILE__)."/connect.inc";

$pc = oci_connect("app[bob]", "ppa", $dbase);
$stmt = oci_parse($pc, "select user from dual");
oci_execute($stmt);
var_dump(oci_fetch_array($stmt, OCI_RETURN_NULLS));
echo "Done\n";
?>

result:
array(2) {
  [0]=>
  string(3) "BOB"
  ["USER"]=>
  string(3) "BOB"
}

From OCI8 1.4.0 PHP 5.3.1 an end-user identity can be passed into the DB with the 
oci_set_client_identifier() function.  This values is used for auditing, VPD and 
monitoring.  See http://www.oracle.com/technetwork/articles/dsl/php-web-auditing-
171451.html