I don't know if anyone else read the article who still actively "has" to develop in php.. but the problem he is asking them to solve with pencil+paper should be

I don't know if anyone else read the article who still actively "has" to develop in php.. but the problem he is asking them to solve with pencil+paper should be totally doable in a few minutes; even for someone fresh to the language as other than maybe the strcmp function you would need no real "library" knowledge. I think it's clear that there are two kinds of "solid" php developers. The ones that have moved on to other languages/paradigms and love it (thus answering "where have they gone") and those who have moved on but still make their bread and butter in php but apply those new paradigms best as they can (cake php, fluent expressions, correct use of OO etc.). The problem with hiring the latter is they are most likely gainfully employed or more interested in doing their own thing. Maybe you should try hiring people with aptitude and providing crash course training as well as further education incentives.