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.
"People" questioning the numbers published by multiple outlets over at least a decade? Who? What data do they have to "conclude that the number is very unreliable?"
Whether PHP runs 77% or 69% of public web sites, how does that offend anyone or make them feel insecure? No one is trying to "prove" anything, there's no race to the one ultimate tech stack that requires winners and losers. You can accept the fact that PHP objectively runs a large majority of public web sites without interpreting that as a threat to your choices, your job, your image of yourself as a professional.
Having so much PHP out there may look like a problem, but programmers attaching their ego and identity to languages and tools and frameworks accounts for a lot more wasted time and crappy code than a popular language that has some obvious and well-known flaws.
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.