[php-src] Issue #12259: Convert built-in function names to camel case

Issue: https://github.com/php/php-src/issues/12259 Author: salimibrohimi ### Description Per [PSR-1](https://www.php-fig.org/psr/psr-1/) function names must be in camelCase (which is great). Can we rename all built-in function names using camelCase? For example, we could rewrite array_map(), str_replace() to arrayMap(), stringReplace() respectively. For backward compatibility leave the old functions too, but mark them as deprecated and remove them in future versions. It would've been great to have function names standardized and intuitive. Right now there are many functions that have short but non-intuitive names like strrev(), strncmp(), strnatcmp() etc.