Jack Herrington is brave: he argues that PHP is at least as scalable as Java, and he does it on O’Reilly’s ONJava Website… His arguments are quite obvious:
Still not convinced? Consider JSR 223, the effort to turn PHP into the front end for J2EE by porting it to Java. If PHP on top of Java is scalable, then why isn’t PHP on top of C?
Sandro Groganz 
To sum it up, Jack says that PHP scales… if you don’t use it (“if you do less”).
To do more and scale better, look at TrustLeap G-WAN:
http://trustleap.com/
Cheers!
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