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. You might 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