The question about which (one) language has implications for code design for all development issues.

you can break an application down into a number of discreet but associated stages. The simplest is 3-tier

User Interface
Business Logic
Data Storage

This is one reason Access rocks (or sucks depending on which way you look at it). You can do all three tiers in a single product.

When working on the web you will usually have 3 separate tiers

HTML for the user interface (ask the questions, display the results)
A database to hold the information (MSSQL or MySQL normally)
A programming language to decide what information should be displayed (ASP/.NET/Perl/Python/PHP)

The big advantage of separated structures is that in theory you can swap one element for another and your application will still work.

e.g. swap your web front end for one written in Access, looking at the same scripts on the web server and your application still works. change the business rules in the code on the server and both the web and client side both change what they display. Change mySQL for MSSQL and the business rules still get the same data.

Of course it is never that simple. You can add another tier and have the business logic held inside the db using stored procedures. Your PHP/ASP code then simply call the SPs, passing through whatever parameters are required. you could introduce ajax to provida and RIA experience which would probably require lots of additional server side code to handle the ajax requests.
You can use a framework (Cake, ruby, Codeigniter etc) for the server side programming which takes away some of the mundane activity like connecting to databases or building web forms and doing validation.

Generally, unless you are going to use Access there isn't one language to learn but several and Access is really three languages anyway (SQL, VBA and GUI forms).

Personally I would learn SQL (microsoft T-SQL or mySQL which is ANSI standard) and PHP and HTML as a starting point as you they are free and you can build either client side or server side applications with these. You would then want to look at javascript/ajax and/or = VB.NET/ C#.NET to complete your knowledge base.

obiron