[Editor's note: OCI8 1.3 should not experience the problem described in this user comment. The first use of such a connection will return an Oracle error which will trigger a cleanup in PHP.  Subsequent persistent connection calls will then succeed.  For high availability you might consider doing consecutive oci_pconnect calls in your script.]
If you connect using oci_pconnect and the connection has logged you off but is still valid, there seems to be no way to re-use that connection. The next time I try oci_pconnect and then perform an oci_execute operation, I get a "ORA-01012: not logged on" warning. This problem remains, even if I close the connection using oci_close. I ended up with the following (rather annoying) code.
<?php
    function getOracleConnection()
    {
      if (!function_exists('oci_pconnect'))
        return false;
      $toReturn = oci_pconnect('user', 'pass', 'db');
      if ($testRes = @oci_parse($toReturn, 'SELECT Count(group_type_code) FROM pvo.group_type'))
        if (@oci_execute($testRes))
          if (@oci_fetch_array($testRes))
            return $toReturn;
      oci_close($toReturn);
      if (!function_exists('oci_connect'))
        return false;
      $toReturn = oci_connect('user', 'pass', 'db');
      if ($testRes = @oci_parse($toReturn, 'SELECT Count(group_type_code) FROM pvo.group_type'))
        if (@oci_execute($testRes))
          if (@oci_fetch_array($testRes))
            return $toReturn;
      oci_close($toReturn);
      if (!function_exists('oci_new_connect'))
        return false;
      $toReturn = oci_new_connect('user', 'pass', 'db');
      if ($testRes = @oci_parse($toReturn, 'SELECT Count(group_type_code) FROM pvo.group_type'))
        if (@oci_execute($testRes))
          if (@oci_fetch_array($testRes))
            return $toReturn;
      oci_close($toReturn);
      return false;
    }
?>