1. Yes, concurrent means at the exact same time. However, if you exceed
the
limits of Access it will print a page to the end user's web browser that
the
server is busy (you may have seen this before). Lastly, unique visitors
would
have nothing to do with the database being accessed unless you are using
database driving templates where the database is being queried on every
page.
2. mySQL is a completely different direction then MS Access. There is
no
application it is just a database like MS SQL. The SQL is a type of
database.
There are a variety of applications out there to manipulate these
databases.
If you are developing these web applications though, there should not be
too
much back-office entering of information, but if there is you may want to
consider developing a front-end for the user base.
3. Why are you switching from ASP/Access to PHP/mySQL? Is there a
reason for
abandoning ASP? If not sometimes it might be also good to consider ASP/MS
SQL
or ASP/mySQL. The learning curve would depend on you though. It is a
different scripting language and a different database language so you are
actually learning two things at once. I wouldn't call this overnight and
would
probably recommend that you get a consult done of your environment to see
if
this is cost effective for your business.
