Using APT

First, note that other related packages may be desired like libapache-mod-php to integrate with Apache 2, and php-pear for PEAR.

Second, before installing a package, it's wise to ensure the package list is up to date. Typically, this is done by running the command apt update.

# apt install php-common libapache2-mod-php php-cli

APT will automatically install the PHP module for Apache 2 and all of its dependencies, and then activate it. Apache should be restarted in order for the changes take place. For example:

# /etc/init.d/apache2 stop
# /etc/init.d/apache2 start

Better control of configuration

In the last section, PHP was installed with only core modules. It's very likely that additional modules will be desired, such as MySQL, cURL, GD, etc. These may also be installed via the apt command.

# apt-cache search php
# apt search php | grep -i mysql
# aptitude search php

The list of packages will include a large number of packages that includes basic PHP components, such as php-cgi, php-cli, and php-dev, as well as many PHP extensions. When extensions are installed, additional packages will be automatically installed as necessary to satisfy the dependencies of those packages.

# apt install php-mysql php-curl

APT will automatically add the appropriate lines to the different php.ini related files like /etc/php/7.4/php.ini, /etc/php/7.4/conf.d/*.ini, etc. and depending on the extension will add entries similar to extension=foo.so. However, restarting the web server (like Apache) is required before these changes take affect.

Common Problems

• If the PHP scripts are not parsing via the web server, then it's likely that PHP was not added to the web server's configuration file, which on Debian may be /etc/apache2/apache2.conf or similar. See the Debian manual for further details.
• If an extension was seemingly installed yet the functions are undefined, be sure that the appropriate ini file is being loaded and/or the web server was restarted after installation.