Technically, PHP and MySQL should be fine for large projects, but many large corporations shun their use because they are open source. This means that, while some excellent support is available via a huge user community, corporate users cannot tie down the supplier with the usual support and maintenance contracts. Instead, these large users turn to DBMSs such as Oracle, where they believe the support will be better assured.
As Sticky's link shows, MySQL 3.23 supports a theoretical maximum table size of 8 million terabytes. This should be OK for most projects
Regards
Patch