Brian Fioca of Jobby recently talked to Owen Bryne, senior software engineer and one of the co-founders of digg.com, about the scalability of PHP on big sites.  It turns out at digg.com, PHP scales really well:

To get a better idea of what was in store for a heavily loaded PHP application, I set up an interview with Owen Byrne, cofounder and Senior Software Engineer at digg.com. From talking with Owen I learned digg.com gets on the order of 200 million page views per month, and they’re able to handle it with only 3 web servers and 8 small database servers (I’ll discuss the reason for so many database servers in the next section). Even better news was that they were able to handle their first year’s worth of growth on a single hosted server like the one I was using. My hardware worries were relieved. The hardware requirements to run high-traffic PHP applications didn’t seem to be more costly than for Java.

Mr. Fioca claims to have more experience working with Java and traditional big iron databases, and so he spends most of the article comparing his Java experience with digg's PHP architecture.  What's really interesting is the tidbit about digg's tremendous traffic being served by only 3 web servers (and 8 database servers). 

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