I have same problem with UTF8 database and UTF8 client.
PDO_OCI should set string buffer length correct to handling UTF8.

Check also oracle OCI documents below:
http://download.oracle.com/docs/cd/B10500_01/server.920/a96529/ch6.htm#1004620

- Database settings
SQL> 
  1  SELECT PARAMETER, VALUE
  2    FROM NLS_DATABASE_PARAMETERS
  3*  WHERE PARAMETER IN ('NLS_CHARACTERSET', 'NLS_NCHAR_CHARACTERSET')

PARAMETER                      VALUE
------------------------------ ------------------------------
NLS_CHARACTERSET               AL32UTF8
NLS_NCHAR_CHARACTERSET         AL16UTF16

- Table for test
CREATE TABLE mytable (col1 NVARCHAR2(20));

- Test data
INSERT INTO mytable VALUES('12345678901234567890'); /* 20 signle byte char */
INSERT INTO mytable VALUES('あいうえおかきくけこさしすせそたちつてと'); /* 20 double byte char, Japanese */

- Test Program
<?php
    print "NLS_LANG=".getenv('NLS_LANG')."\n\n";

    $db = new PDO("oci:dbname=//instance1.cf9klgqzy0gu.ap-northeast-1.rds.amazonaws.com:3306/mydb;charset=AL32UTF8", "user", "pass");
    $db->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
    $stmt = $db->prepare("SELECT * FROM mytable");
    $stmt->execute();
    var_dump($stmt->fetchAll(PDO::FETCH_ASSOC));
?>
- Test Program Output
# php ocitest.php 
  
NLS_LANG=Japanese_Japan.AL32UTF8


Warning: PDOStatement::fetchAll(): column 0 data was too large for buffer and was truncated to fit it in /root/ocitest.php on line 9
array(2) {
  [0]=>
  array(1) {
    ["COL1"]=>
    string(20) "12345678901234567890"
  }
  [1]=>
  array(1) {
    ["COL1"]=>
    string(40) "あいうえおかきくけこさしす
  }
}

- Client Environment
# uname -a
Linux localhost.localdomain 2.6.18-194.el5 #1 SMP Fri Apr 2 14:58:35 EDT 2010 i686 i686 i386 GNU/Linux
# rpm -qa | grep oracle
oracle-instantclient11.2-devel-11.2.0.2.0-1
oracle-instantclient11.2-sqlplus-11.2.0.2.0-1
oracle-instantclient11.2-basic-11.2.0.2.0-1
# php -v
PHP 5.3.6 (cli) (built: Aug  5 2011 09:15:02) 
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies

- Suggest fix
Patch to oci_statement.c

# diff -u oci_statement.c oci_statement.c.new 
--- oci_statement.c     2011-08-06 01:07:53.000000000 -0700
+++ oci_statement.c.new 2011-08-06 01:08:03.000000000 -0700
@@ -529,7 +529,7 @@
                        (param, OCI_DTYPE_PARAM, &colname, &namelen, OCI_ATTR_NAME, S->err));
 
        col->precision = scale;
-       col->maxlen = data_size;
+       col->maxlen = ( data_size + 1 ) * sizeof(utext);
        col->namelen = namelen;
        col->name = estrndup((char *)colname, namelen);

i've issued the same bug with different version of oracle client (10.2) and the 
patch i've resolved the problem.

This bug is nearly exact duplicate of bug #35003:

https://bugs.php.net/bug.php?id=35003

Then only difference is that this one is still open (second one: closed) and 
this contains a solution, how to fix this bug. Even though, it requires to 
change ONE line and recompile sources, no one at PHP Dev Team took care to fix 
it. Even though, this is really huge bug, as it completely prevents using PHP + 
Oracle on UTF-8 encode webpages / databases.

The last change is, that this bug issue remains open for "only" 1,5 year, while 
second one, closed, but still not fixed, remains untough for SEVEN years (since 
initial report).

See PR at https://github.com/php/php-src/pull/59

See PR https://github.com/php/php-src/pull/542

Hi,

This bug is annoying. There are fixes for it. 
Why arent they merged?

Sven