A small warning about the double metaphone function/class implemented in php: it's slow. Very slow.
On my Mac OS X box it takes roughly 4 seconds to calculate the double_metaphone of a given string 1,000 times and set a variable to that value. Given the exact same code, the built in metaphone function takes a mere 0.03 seconds. That's a huge difference. For reference, soundex takes about 0.015 seconds.
I had dreams of ranking my person database search results with double_metaphone, but with searches yielding a few thousand results sometimes, this isn't terribly reasonable. I'm back to basic metaphone functions.
I'd suggest anyone considering the use of the double_metaphone functionality weigh the slight improvement in results with the fact it's 100 times slower than the php function metaphone. I believe this difference is probably due mostly to implementation and optimization (php code vs. optimized internal php function).
Anybody up for writing a double_metaphone function for php? Maybe php 5?