On DeveloperWorld today there's a new article talking about performance concerns and PHP application (and how your data source might be the problem).
The power of PHP and an RDBMS is the ability to nail the major features of an application with cheaply paid developers in a record amount of time. Unfortunately, the default runtime environment used by PHP is simply an unscalable mess. [...] The truth is that if you have enough servers and enough database servers, you don't have contention. [...] As it turns out, there's a modern solution to the problem: the cloud plus NoSQL. Cloud infrastructure gives us the ability to spin up enough servers, and a NoSQL database enables us to shard our data effectively.
They talk some about why they think PHP's runtine environment is "a dog" based on the non-native pooling of database connections and the lack of a thread-safe environment.
The bottom line: PHP applications are a load on the database due to the constraints of the concurrency model.
He points to the cloud architecture and NoSQL databases as solutions to the scalability problem, providing more scalable resources and flexible data sources.