Best way to know if there is a row

I have a bunch of queries in a system I'm finishing, and I bumped with a
question on performace.
Which is the best way to solve this:

I need to know if there is at least one row in the relation that comes from a
determinated query. Which is the best way to do this:

(a) Build the query with "SELECT * ..." and after executing the query see if
numRows()>0
(b) Build the query with "SELECT count(*) ...", fetch the row and see if
count>0

I'm working with (a) because I see it better in performace, but I wanted to be
sure the numRows() will actually give me the exact amount of rows (had some
problems in the past with Informix).

The aplication is written in PHP.

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Martín Marqués        | select 'mmarques' || '@' || 'unl.edu.ar'
Centro de Telematica  |  DBA, Programador, Administrador            Universidad Nacional                 del Litoral
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