People have already questioned the validity of this number. Do a search and you'll find people looking into this and conclude that the number is very unreliable. Whether you agree or not is up to you.
Also I want to point out that almost any time people quote number about PHP's popularity, this is the only number, which is strange -- for metrics like iOS market share you can always find multiple numbers from multiple sources which don't fully agree with each other but are within a certain range. Not for this PHP number. In other words, w3tech's number is not cross validated by any other source. I wouldn't use it to "prove" anything.
Also I want to point out that almost any time people quote number about PHP's popularity, this is the only number, which is strange -- for metrics like iOS market share you can always find multiple numbers from multiple sources which don't fully agree with each other but are within a certain range. Not for this PHP number. In other words, w3tech's number is not cross validated by any other source. I wouldn't use it to "prove" anything.