Re. the Tiobe index: " The popular search engines Google, MSN, Yahoo!, Wikipedia and YouTube are used to calculate the ratings".

So lots of script-kiddies are Googling for PHP instead of getting formal training, buying a good book or even just RTFM. If a language has good built-in documentation then it will perform badly on Tiobe, little or no documentation means it does well. Tiobe is a useful indicator, but only of what people are doing searches for.