You can make PHP throw on notices if you like.

The language/runtime is very configurable. Some codebases will set strict_type=1 plus error_reporting(E_ALL) eliminating many instances of these kind of problems / footguns.

Sure, classic CGI is one process per request, but even on php-fpm or apache prefork/mod_php each process will only work for a given maximum number of requests, thus refreshing periodicaly. Also, if any of these workers is killed (say, by the OOM-killer), none of the concurrent requests will be affected. Also, the overhead of parsing is almost zero when you have the opcache extension loaded.

I'm not a huge fan of the language, but the PHP runtime wins me by its simplicity and robustness. I work daily on a codebase that has hundreds of millions of page hits per day and 100k QPS during peak hours running on LAMP with a very few servers. We face challenges like everyone else does, but we never had outages because of the languange/runtime.