\set variant for use in regular sql commands

I need to use the \set command in regular sql (I am running postgres queries
through php and it does not seem to support the \ commands in general). How
can I set a variable with regular sql?

I want to be able to do the following:

\set testvar '1,2,3,4,5'
\set testvar '6,7,8,':testvar

Which is then used in: select * from table where id in (:testvar);

Thanks,

--Jeff