Based on your comment, I don't think it is installed correctly. Make sure you see the section in phpinfo (), for example:
memcached memcached support enabled Version xxx libmemcached version x.xx
1) Get libmemcached
2) Configure the package
3) Install sudo make install libmemcached
4) Try installing with installation using sudo pecl install memcached
5) make sure you have extension = memcached.so in php.ini
Restart apache
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