Database sybase

The following select works using slq advantage and returns values but when I use php I get nothing and NO errors either.

SELECT
    table_name.num_a,
    table_name.num_b
from table_name
where table_name.num_b <> null and
      table_name.num_a=1111

I also tried

SELECT
    table_name.num_a,
    table_name.num_b
from table_name
where table_name.num_b != null and
      table_name.num_a=1111

In both cases I get no results but when using SQL adavantage from sybase I get the expected results.

The problems comes with the line
table_name.num_b != null 

It seems like a bug.  Please advise.

Thank you