Quality > quantity (just because a language is widely used doesn't mean it's well designed), a huge amount of this quantity are WordPress blogs, MediaWiki sites, or relatively low traffic e-commerce sites. It is not the dominant language of "tons of websites with huge traffic and usage."[1] [Facebook and Wikipedia are the notable outliers...so I'll admit I shouldn't have said sole outlier]. The "quantity" argument is also weakened by the fact that websites are not the only things programming languages are used for, and in terms of overall popularity it's use is a fraction of that of C or Java.[2] PHP also has far less job openings than Java or C# and only slightly more than Python[3] (I imagine Python will eclipse PHP fairly soon).
If you're interested in why so many developers consider PHP a poorly designed language I'd suggest reading PHP: a fractal of bad design[4]
[1] https://en.wikipedia.org/wiki/Programming_languages_used_in_...
[2] http://www.tiobe.com/index.php/content/paperinfo/tpci/index....
[3] https://gooroo.io/GoorooTHINK/Article/16191/Which-language-w...
[4] http://eev.ee/blog/2012/04/09/php-a-fractal-of-bad-design/