I am doing some work on an asp website that uses Access as its database.
Recently, the site has been having unexplained downtime, and we are wondering if making a whole new site, and switching to PHP with mySQL would bring a big performance boost or not.
It is a fairly busy site with about max 80 people connected at once.
It's impossible to answer "is it worth"-type questions definitively.
However, for your situation, I'd say the first thing you need to do is turn the "unexplained downtime" into "explained downtime". Figure out what's causing the problem. If it turns out that it's a hardware error on the server, you're going to rewrite the whole site and it'll still keep going down.
Then, if you determine that it's the Access db causing the problem, the next thing I'd do is just transition that to MySQL, but leave the site in ASP. If you're just concerned about the database, why are you also considering a PHP rewrite?
I would say yes, it would. I don't think access was ever really intended to power data driven websites, and it isn't nearly as powerful as a database engine like MySQL or SQL Server.
I would absolutely get away from Microsoft Access. Over time I have learned that exclusive opening and intermittent locks by the web user can cause Access to not be available and that no good solution exists to fix this.
.mdb files are notorious for getting corrupt in my experience.
I use Access as the front end tool for visually building queries, but never for the repository unless it is very trivial and never for a web app (never again, anyway.)
I don't know that PHP would help you. But changing the DB will help you.
MySQL provides much better performance than Access. MySQL is capable of handling a lot more concurrent connections than Access.
I'm surprised to see Access handling 80 connections - I've normally seen it fall over after the first dozen or so.
Access is intended for small workgroups. It only supports 2GB of data per database and isn't really designed for 80 concurrent users. I would strongly recommend switching a more robust RDBMS.
Another thing to worry about with Access is that because it was not designed to handle a lot of users at once efficiently, data corruption could rear it's ugly head. MySQL or SQL Server can handle a ton of users much better. However, I agree with Chad Birch that you should keep the site in ASP to avoid the headache of transitioning to PHP.
SQL Server/Express would be a large improvement or just a SQL .mdf File if your provider doesn't have SQL Server.