This is actually acceptable behavior for odbc_num_rows().  The PHP
documentation says as much:
http://de.php.net/manual/en/function.odbc-num-rows.php

I can't comment on the reasons why DB2 cannot return the number of rows
returned by the first statement.

There are some terrible suggestions in the comments to the function how to
get it to return some semi-reliable results (like running the query twice).  
Maybe you could first explain what you need the row-count for.  Then it
might be possible to give you a better advise.

The only reason mysql_num_rows() returns the number of rows from a
SELECT statement is because MySQL immediately returns ALL of the rows
to the client for a mysql_query() call. Yes, this means that it's a bad
idea to do a SELECT that returns a million rows against MySQL. (Note
that the MySQL interface in PHP recently introduced
mysql_unbuffered_query() which returns only one row at a time -- and
surprise surprise, mysql_num_rows() will return 0 for a SELECT
statement when you call mysql_unbuffered_query().)

You have several options for getting the number of rows from a SELECT
statement in DB2:
1. Return all of the rows from the SELECT statement yourself, basically
explicitly doing the same thing that MySQL does under the covers:

while ($result_set[] = db2_fetch_array($stmt)) {}
$num_rows = count($result_set);

2. Very often, mysql_num_rows() is used only to determine whether any
results were returned from the SELECT statement. In this case, just go
ahead and issue your first db2_fetch_*() call -- if it returns FALSE,
then you know that there are no rows in the result set.

3. Use a scrollable cursor for your SELECT statement -- this uses more
resources on the database server but will allow db2_num_rows() to
return the right number of rows.

$stmt = db2_prepare($conn, 'SELECT firstnme FROM employee",
array("cursor" => DB2_SCROLLABLE));
db2_execute($stmt);
$num_rows = db2_num_rows($stmt);

Oh yeah, and please stay away from Unified ODBC for all of the reasons
mentioned in my article (the --with-ibm-db2 compile option) and use the
ibm_db2 extension instead (http://php.net/ibm_db2 or install the free
Zend Core for IBM from http://zend.com/core/ibm/ for precompiled PHP +
ibm_db2 extension). You'll have much more functionality and better
performance.