As a consultant I work with DB2, Oracle, SQL Server, MySQL and PostgreSQL. Which DB to use is <i>usually</i> not my decision, governed mostly by the type of OS and platform.
That said, my personal stuff runs on Linux+Apache+PHP, and my choice of DB is...
<b>PostgreSQL all the way!</b>
I create web site infrastructures that let people maintain 99% of their web site's content through a web browser interface (ie "html forms"). As a result, 99% of the web site content comes from the DB. <I>Every page request hits the db many times.</I>
My old hosting company only had mySQL so I used that and it worked pretty good. When I switched to another hosting company, they offered postgreSQL so I converted. I have been sold on it ever since!
The server configurations of the two hosting companies are comparable, but it just seems to me that postgreSQL runs smoother, especially on the more sophisticated joins and on larger tables.
So far I have never lost any data (knock on wood) and have never even had to restart the postmaster.
You will no doubt get many that prefer mySQL, and I may get flamed for this post, but this is just my experience, preference, and 2cents worth.
-- Rich