Why is the memcached extension not installed?

Question

PHP

1. Installed memcached: brew install memcached
result: ==> Summary
/usr/local/Cellar/memcached/1.5.16_1: 11 files, 203.5KB
2. In php.ini set the line extension = memcached.so
3. Restart the server, call in script:
output

PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/lib/php/extensions/no-deb
ug-non-zts-20160303/memcached.so' - dlopen(/usr/lib/php/extensions/no-debug-non-zts-20160303/memcached.so, 0x0009):
dlopen(): file not found: /usr/lib/php/extensions/no-debug-non-zts-20160303/memcached.so in Unknown on line 0

Indeed, I go to /usr/lib/php/extensions/no-debug-non-zts-20160303/memcached.so and there is no memcached.so. So the library is not installed? and how to install it?
Searching for mdfind memcached.so -name returns nothing.
Given:
Embedded Web Server, macOS