>> My question is, when do we use the typical select statement and when
>> do we use the "execute procedure" approach in calling the procedure
>> from PHP?

There are 2 types of the stored procedures:
1-The stored procedures which return the data sets
to use: select * from <SP_Name> (param1,param2)
2-The stored procedures which modify data
to use: execute <SP_Name> (param1,param2)

Read InterBase v6 MediaKit docs:
Data Definition Guide http://ibase.ru/v6/doc/datadef.zip
Language Reference http://ibase.ru/v6/doc/langref.zip

and FireBird docs:
http://www.ibphoenix.com/downloads/Firebird_v15pt1_ReleaseNotes.pdf
--
Best regards,
Dmitry Kalugin
Delphi/FireBird software developer
Ukraine, Donetsk