Oracle Database Discussions

Hello guys,
i have a look at the some php scripts which are connecting to an oracle database and it looks like the following example.

-> $db = @ora_logon("scott@testdb29","tiger")
Ok here is a connect done to oracle

-> $curs = ora_open($db);
Ok a cursor is opened

-> $sql = "SELECT * FROM dept"; ora_parse($curs,$sql)
Ok the sql statement is parsed, with the previous opened cursor

-> ora_exec($curs);
Statement is executed

-> while (ora_fetch_into($curs, $results)) { $results[0]; }
Fetches from the select data

In which step does oracle starts to read the data (logical i/o or phyiscal i/o)?
Does it start at execute or does it start at the fetch?

When it starts at execute... what happens if i don't fetch the data? for example if oracle had to sort the data in the pga...

When it doesn't start at execute... what is done on the execute command?

Are these steps the same in every client (for example sqlplus) or can it vary?

Thanks and Regards
Stefan