在尝试了所有方法之后，我想出了这个解决方案

当默认情况下PHP 7.0 FPM未启用时，将出现此错误。

要在Apache2中启用PHP 7.0 FPM，运行:

 sudo 2enmod proxy_fcgi setenvif
 sudo a2enconf php7.0-fpm
 systemctl restart apache2

如果您使用的是php7.4，在上面的命令中输入7.4而不是7.0。

这种情况又发生在我身上，服务器下载html文件，而不是处理。我已经有一段时间没有在电脑上使用web服务器apache了，同时Ubuntu从最初安装的LTS更新了两个版本。现在是这样

$ cat /etc/issue
Ubuntu 16.04 LTS

php是这样工作的:

$ sudo apt-get install lamp-server^
$ sudo a2enmod php7.0
$ sudo service apache2 restart 

web服务器现在正在解析php。也许现在要更新一些网页，因为php7.0现在运行在php5之前的地方。哦。

我发布这个答案是因为我的Virtualmin/Webmin管理界面决定禁用我的PHP引擎是一个好主意。我花了一段时间才找到解决方案，所以我想和你们分享一下:

同时，确保你的网站配置文件中没有任何与这个特定主机或虚拟主机相关的php_admin_value在它们中关闭PHP，就像这样:

php_admin_value engine Off

有疑问的时候，评论一下…

# php_admin_value engine Off

并重新启动您的web服务器。

听起来你的配置好像有问题，下面是一些你可以检查的东西:

Make sure that PHP is installed and running correctly. This may sound silly, but you never know. An easy way to check is to run php -v from a command line and see if returns version information or any errors. Make sure that the PHP module is listed and uncommented inside of your Apache's httpd.conf This should be something like LoadModule php5_module "c:/php/php5apache2_2.dll" in the file. Search for LoadModule php, and make sure that there is no comment (;) in front of it. Make sure that Apache's httpd.conf file has the PHP MIME type in it. This should be something like AddType application/x-httpd-php .php. This tells Apache to run .php files as PHP. Search for AddType, and then make sure there is an entry for PHP, and that it is uncommented. Make sure your file has the .php extension on it, or whichever extension specified in the MIME definition in point #3, otherwise it will not be executed as PHP. Make sure you are not using short tags in the PHP file (<?), these are not enabled on all servers by default and their use is discouraged. Use <?php instead (or enable short tags in your php.ini with short_open_tag=On if you have code that relies on them). Make sure you are accessing your file over your webserver using an URL like http://localhost/file.php not via local file access file://localhost/www/file.php

最后，查看PHP手册以获得更多的设置技巧。

使用Debian 10安装Apache + php7测试的最简单方法:

apt-get update -y
apt-get install apache2 php7.0 libapache2-mod-php  -y
sudo service apache2 restart

检查你为php安装的所有包，使用:

yum list installed | grep remi

在你的机器上安装所有相关的php包，尤其是php-devel。