SQL and DB Connection Question (Interfacing with PHP)

i've searched high and low on the net and have been
unable to find a good source for how to handle php
variables and constants in the sql string. does
anyone have a link to a good site? i'm as much
interested in the *why* as in the actual syntax.

there's lots of information on how to handle static
information... even in my plethora of books... but
who really does that? it would seem to me that using
forms to enter data would be much more prevailent and
that's where the tutorials should focus.

another issue i'm debating right now is how to handle
the database connection parameters (dbname, host,
username, pw). should that be included in a function?
and include? both?

does anyone have a link to a tutorial or an example of
a good methodology?

tia...

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