Over here you can find a (slightly outdated) presentation given about Flickr. Apparently it is implemented in PHP 4 using MySQL as a database. Some interesting stats from the presentation:
- 60,000 lines of PHP4 code
- 60,000 lines of templates
- 70 custom smarty functions/modifiers
- 25,000 DB transactions/second at peak
- 1000 pages per second at peak
Note these are outdated probably, but it’s nice to know for an indication. Somehow you expect that these kinds of sites use very fancy C++, Java, or whatever languages or frameworks, but that does not have to be the case.
See the presentation here (PDF).