In a new post to his blog Gaylord Aulke talks about some of the "soft facts" that make PHP different from Java:

Besides the usual aspects: scripting vs. compiled and in-process vs. seperate process and Multithreading etc., i think there are some "soft facts" that might be even more important for commercial software development.

He points out things like differences in object lifetimes, defined structures in applications and the "not invented here" syndrome that he thinks PHP suffers more from.

Anyway, the result is: Even though there are not so many standards in the PHP world, successful developers have a common understanding about the do's and don'ts in PHP. [...] PHP appears more predictable than other programming languages.