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.
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.