ubuntu下多版本PHP切换：

root@k8s-server:$ update-alternatives --config php 
There are 2 choices for the alternative php (providing /usr/bin/php).

  Selection    Path             Priority   Status
------------------------------------------------------------
  0            /usr/bin/php8.1   81        auto mode
* 1            /usr/bin/php7.4   74        manual mode
  2            /usr/bin/php8.1   81        manual mode
  
Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/php7.4 to provide /usr/bin/php (php) in manual mode

然后查看/usr/bin/php，其对应的软连接为 /etc/alternatives/php，说明是可选择的
/usr/bin/php---->/etc/alternatives/php
  
查看/etc/alternatives/php，发现又会指向真正设置的版本:
$ ls -l /etc/alternatives/php
lrwxrwxrwx 1 root root 15 12月 12 09:39 /etc/alternatives/php -> /usr/bin/php7.4
  
或者通过 readlink 命令查看:
$ readlink -f /etc/alternatives/php
/usr/bin/php7.4

不同版本php状态查看:

$ systemctl status php8.1-fpm
$ systemctl status php7.4-fpm