PHP版本：PHP 7.3.16
LINUX版本：7.6

安装成功的样子：

php -r "phpinfo();" | grep memcached
memcached
memcached support => enabled
libmemcached version => 1.0.16
memcached.compression_factor => 1.3 => 1.3
memcached.compression_threshold => 2000 => 2000
memcached.compression_type => fastlz => fastlz
memcached.default_binary_protocol => Off => Off
memcached.default_connect_timeout => 0 => 0
memcached.default_consistent_hash => Off => Off
memcached.serializer => php => php
memcached.sess_binary_protocol => Off => Off
memcached.sess_connect_timeout => 0 => 0
memcached.sess_consistent_hash => On => On
memcached.sess_consistent_hash_type => ketama => ketama
memcached.sess_lock_expire => 0 => 0
memcached.sess_lock_max_wait => not set => not set
memcached.sess_lock_retries => 5 => 5
memcached.sess_lock_wait => not set => not set
memcached.sess_lock_wait_max => 150 => 150
memcached.sess_lock_wait_min => 150 => 150
memcached.sess_locking => On => On
memcached.sess_number_of_replicas => 0 => 0
memcached.sess_persistent => Off => Off
memcached.sess_prefix => memc.sess.key. => memc.sess.key.
memcached.sess_randomize_replica_read => Off => Off
memcached.sess_remove_failed_servers => Off => Off
memcached.sess_sasl_password => no value => no value
memcached.sess_sasl_username => no value => no value
memcached.sess_server_failure_limit => 0 => 0
memcached.store_retry_count => 2 => 2
Registered save handlers => files user memcache memcached

安装步骤：

# 首先需要安装libmemcached库
yum -y install libmemcached libmemcached-devel
# 下载memcached扩展，两个地址任选一
wget https://github.com/php-memcached-dev/php-memcached/archive/php7.zip
wget https://pecl.php.net/get/memcached-3.1.3.tgz
# 解压
unzip php7.zip
or
tar zxvf memcached-3.1.3.tgz
# 进入到解压后目录
# 准备安装
export PHP_AUTOCONF=/usr/bin/autoconf
export PHP_PREFIX=/opt/php
/usr/local/php/bin/phpize
# 配置
./configure --with-php-config=/opt/php/bin/php-config
make
make install
# 在php.ini中添加memcached扩展
extension=memcached.so
# 可以使用下面二种方式中的一种(具体看php的安装方式)
service php-fpm restart
systemctl reload php-fpm