Description:
------------
Supplying the constant OCI_DESCRIBE_ONLY as the second 
argument (mode) to oci_execute() results in a warning,
and oci_execute() returning false.

Reproduce code:
---------------
The bug is caused by these lines of code in
  php-5.1.2/ext/oci8/oci_statement.c
at line 1351 in the definition of
  php_oci_statement_execute():


   switch (mode) {
     case OCI_COMMIT_ON_SUCCESS:
     case OCI_DEFAULT:
       /* only these two are allowed */
       break;
     default:
       php_error_docref(NULL TSRMLS_CC, E_WARNING,  
            "Invalid execute mode given: %d", mode);
       return 1;
       break;
   }


Expected result:
----------------
I would expect oci_execute() to pass the mode 
OCI_DESCRIBE_ONLY on to the underlying OCI8 call.

I think it is very bad that this non- backward-
compatible change has been made without being documented 
or announced anywhere. I am responsible for an 
application that is used by several thousands of users 
daily that relies on OCI_DESCRIBE_ONLY working.