Problem either with PostgreSQL or with PHP

Hi!

I have written this function in PL/pgSQL,
CREATE OR REPLACE FUNCTION MakeSimpleReservation(integer, integer, integer, text, text) RETURNS boolean ...

In my PHP script, I have this code:
 $start_ts = '2010-04-12 11:00:00';
 $end_ts   = '2010-04-12 14:00:00';

 $update = pg_query($yhteys, "SELECT MakeSimpleReservation(2, 3, 1, '{$start_ts}' , '{$end_ts}');");

On psql, I can run a command such as

select * from MakeSimpleReservation(2, 30, 1, '2010-04-12 11:00:00', '2010-04-12 14:00:00');

without any problems, but when I try to run it via PHP, I get this error message:

ERROR: function makesimplereservation(integer, integer, integer, unknown, unknown) does not exist LINE 1: SELECT
MakeSimpleReservation(2,3, 1, '2010-04-12 11:00:00' ... ^ HINT: No function matches the given name and argument types.
Youmight need to add explicit type casts. 

For some reason, the last two function parameters are not recognized as strings. Is this a problem with PHP or with
PostgreSQL?Can anyone give me advice how to fix this? 

Tuo