按照本教程在 php 版本之间切换：（7.2 和 5.6）：
http://mehedi.info/2017/06/07/use-php-5-6-php-7-0-alongside-one-another-ubuntu/
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php5.6 php5.6-mysql php5.6-mbstring libapache2-mod-php5.6
从 php5.6 到 php7.2：
sudo a2dismod php5.6 ; sudo a2enmod php7.2 ; sudo service apache2 restart
sudo update-alternatives --set php /usr/bin/php7.2
从 php7.2 到 php5.6：
sudo a2dismod php7.2 ; sudo a2enmod php5.6 ; sudo service apache2 restart
sudo update-alternatives --set php /usr/bin/php5.6
sudo a2dismod php7.2 ; sudo a2enmod php5.6 ; sudo service apache2 restart在终端中运行时出现此错误：
模块 php7.2 已禁用 考虑 php5.6 的依赖 mpm_prefork：考虑冲突 mpm_prefork 的 mpm_event：考虑 mpm_prefork 的冲突 mpm_worker：模块 mpm_prefork 已启用 考虑 php5.6 的冲突 php5：模块 php5.6 已启用 apache2.service 的作业失败因为控制过程以错误代码退出。有关详细信息，请参阅“systemctl status apache2.service”和“journalctl -xe”。
是我从中得到的systemctl status apache2.service：
● apache2.service - The Apache HTTP Server Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled) Drop-In: /lib/systemd/system/apache2.service.d
└─apache2-systemd.conf Active: failed (Result: exit-code) since Mon 2018-03-26 10:56:51 IDT; 38min ago Process: 18615 ExecStop=/usr/sbin/apachectl stop (code=exited, status=139) Process: 18891 ExecStart=/usr/sbin/apachectl start (code=exited, status=139) Main PID: 18540 (code=exited, status=0/SUCCESS)
Mar 26 10:56:51 Company systemd[1]: Starting The Apache HTTP Server...
Mar 26 10:56:51 Company apachectl[18891]: Segmentation fault (core dumped)
Mar 26 10:56:51 Company apachectl[18891]: Action 'start' failed.
Mar 26 10:56:51 Company apachectl[18891]: The Apache error log may have more information.
Mar 26 10:56:51 Company systemd[1]: apache2.service: Control process exited, code=exited status=139
Mar 26 10:56:51 Company systemd[1]: Failed to start The Apache HTTP Server.
Mar 26 10:56:51 Company systemd[1]: apache2.service: Unit entered failed state.
Mar 26 10:56:51 Company systemd[1]: apache2.service: Failed with result 'exit-code'.
这是来自systemctl status apache2.service：
--
-- The start-up result is done.
Mar 26 11:33:45 Company nm-dispatcher[20168]: req:1 'dhcp4-change' [wlp3s0]: new request (1 scripts)
Mar 26 11:33:45 Company nm-dispatcher[20168]: req:1 'dhcp4-change' [wlp3s0]: start running ordered s
Mar 26 11:33:45 Company kernel: userif-4: sent link down event.
Mar 26 11:33:45 Company kernel: userif-4: sent link up event.
Mar 26 11:33:52 Company vmnet-dhcpd[2740]: DHCPREQUEST for 192.168.90.128 from 00:0c:29:71:c5:c9 via
Mar 26 11:33:52 Company vmnet-dhcpd[2740]: DHCPACK on 192.168.90.128 to 00:0c:29:71:c5:c9 via vmnet8
Mar 26 11:34:01 Company CRON[20179]: pam_unix(cron:session): session opened for user username by (uid=0
Mar 26 11:34:01 Company CRON[20180]: (username) CMD (/home/username/collectit/bin/collectit.sh >>/tmp/coll
Mar 26 11:34:01 Company CRON[20179]: (CRON) info (No MTA installed, discarding output)
Mar 26 11:34:01 Company CRON[20179]: pam_unix(cron:session): session closed for user username
Mar 26 11:35:01 Company CRON[20202]: pam_unix(cron:session): session opened for user username by (uid=0
Mar 26 11:35:01 Company CRON[20203]: (username) CMD (/home/username/collectit/bin/collectit.sh >>/tmp/coll
Mar 26 11:35:01 Company CRON[20202]: (CRON) info (No MTA installed, discarding output)
Mar 26 11:35:01 Company CRON[20202]: pam_unix(cron:session): session closed for user username
Mar 26 11:36:01 Company CRON[20235]: pam_unix(cron:session): session opened for user username by (uid=0
Mar 26 11:36:01 Company CRON[20236]: (username) CMD (/home/username/collectit/bin/collectit.sh >>/tmp/coll
Mar 26 11:36:01 Company CRON[20235]: (CRON) info (No MTA installed, discarding output)
Mar 26 11:36:01 Company CRON[20235]: pam_unix(cron:session): session closed for user username
我已经用干净的新操作系统（虚拟机）测试了确切的过程，并得到了完全相同的错误。