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Portability is the key to scalable applications. USers can run the same application on a low-scale Linux box and on a high-end Solaris machine, without worrying about platform-specific features. Also, PHP interfaces transparently with clustering solutions from simple Round Robin IP clusters to advanced Cisco solutions.
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The broad platform support can be directly attributed to the fact that PHP is distributed with full source code. Anyone with the necessary skills can port PHP to a new operating system. The resulting modifications to the core system can then be contributed back to the community.