I've been developing web applications for a while and have been using coldfusion and access or mysql. I'm about to undertake a project that could could have tens of thousands of users accessing it. It will all be dynamic content.

This project will be developed in php(I'm a bit new to this).
What database is best for this? (Out of all of them, but I like free ones personaly)

PostgreSQL is open-source (free) and is certainly able to handle the kind of scale you are talking about. It is consistently well-reviewed and has a very active user/developer base. See their website. Also, there was recently an interview with a fellow who runs a web music site - he advocated using PostgreSQL with Ruby on Rails (rather than PHP). There has been an interesting discussion of developments in PHP on the PostgreSQL general mailing list.

There are several constraints, in no order

1) what do the ISP's you are considering using have, support and reccommend. There is little point in designing an app and finding that you preferred or selected ISP isn't familiar with it. They have a resposnsibility to tie te db down to make sure that your data is secure. Granted you may take the view trhat you will develop first and then find an ISP whihc matches those requirements.

2) what are you familiar with. personally I think its a fairly arbitary factor, after all SQL, is SQL, (accepting that there are diffferences in implementation. Access (strictly speaking JET) wouldn't be my choice for a web site, but I suspect that may be prejudice. I have yet to see any defintivie figures that suggest JET is innappropriate for use as a web db, but I'd suspect it would keel over wth medium to heavy use. The choice of server db could be any of the server products, although if you are on a budget then POSTGRES or MySQL are both candidates.

3) if you are devloping the app then the scripting language choices seem to be PHP, ASP or something like coldfusion - again some of the choice is going to be dictated by your ISPs. If you are just learning web development then personally I'd stick to a well known scripting technology, if for no other reason than there are likely to be more people out there who could assist, there may aslo be more books, and equally there may be more freely available classes contributed by developers to the community. There is no point in rewriting what someone else has already done. PHP has several contenders PEAR and sites such as PHPClasses.

4) Versions: if you are using something like MySQL as the db, or PHP as the user interface make sure you are usign the same or compatible versions as likely ISP's are going to offer. Many ISP's still use MySQL 3.23.xx - it is pointless developing something that runs fine in your development environment that can't run on the live server as it desn't support that feature set.

5) is this site a career, or is developing a career - if the latter then possibly your skills are more marketable if you choose ASP over PHP and SQL Server over MySQL or POSTGRES.

As you have experience of coldfusion I'd of thought that would be the natural choice for you. As regards to the choice of db personally I'd go for MySQL, but thats becasue of the 2 free SQL servers candidates its the one I've used - I really don't think it matters which one you do choose. If you do go for MySQL, then I'd suggest your seriously look at using version 5 if your candiate ISP's can support it.
HTH

HTH

I would agree with Healdem's advice in general - well considered comments. However, there may be some concern regarding MySQL with Oracle's recent purchases of the components that MySQL uses for transactions (SleepyCat and, I think, InnoDB). One wonders what Oracle's strategy is, but it looks like they may be trying to back MySQL into a corner. There was also concern whether Oracle's actions might affect PHP.