Zend.com: Under The Hood of PHP 4

"By the time you read this article, PHP 4.0.0 is in final release stages. Since PHP 4 is by far the most advanced version of PHP, I wanted to share with you an inner look at what makes this open source freeware tick. Many of the most important features of PHP 4, such as the replacement of the scripting engine and modularization, are transparent to end users. Nonetheless, these are exactly the features that make the difference between a stable, efficient and reliable Web scripting solution, and a less stable solution...."

"If PHP 3 was so great and popular, what in the world made us write it from scratch, all over, again? In simple terms, we weren't being creative enough. While we did switch to a much reliable approach in our parser (which analyzed the entire script, instead of line-by-line), we didn't question the very nature of PHP, which was: "execute the code while reading it."

"PHP, throughout all of its versions, always "understood" (parsed) and executed the code simultaneously. This was one of the main reasons PHP was so quick with short, simple scripts; there was minimal startup time and it included simple data structures necessary for the implementation. On the other hand, this meant that PHP became less and less efficient as scripts began to be more and more complex, due to the need to repeatedly parse the same parts of the file in many cases, such as loops or repetitive function calls."

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