PHP is a server-side programming language. Common usage includes writing dynamic webpages by loading content server-side before serving to the web client. Traditionally PHP is processed by a PHP handler interfaced from the web server using CGI (protocol/interface for webservers to communicate with), or as standalone interpreters, e.g. mod_php in Apache web servers.

PHP-FPM is a PHP handler daemon that runs as a service instead, which modern web servers like nginx can pass PHP scripts to for server-side processing. This architecture tends to be a bit more lightweight and fast due to it's FastCGI-based implementation.

See this for differences between PHP handler types, and this for differences between CGI and FastCGI.

Installation for php-fpm (usually in combination with nginx) on Ubuntu is performed using:

sudo apt install php-fpm

This method installs only the latest PHP (8.1 as of 2022-12-26), which may cause compatibility issues with older software. For older versions, e.g. 7.4, load it from another repository:

sudo apt-get install software-properties-common
sudo add-apt-repository -y ppa:ondrej/php
sudo apt update
sudo apt install php7.4-fpm

To check the status of the PHP service:

sudo systemctl status php8.1-fpm

Extensions for php is typically installed via the same package manager, with the name of the extension appended to php-, e.g. php-xml.

The behaviour of PHP is modifiable by editing the /etc/php/X.X/php.ini config. Some common values pertaining to script execution and file uploads one might consider adjusting (listed are the defaults):

max_execution_time = 30    # per script
max_input_time = 60        # per script, for parsing data
memory_limit = 128M        # per script
post_max_size = 8M         # size of POST request allowable
file_uploads = On          # HTTP file uploads
upload_max_filesize = 2M   # size of uploaded files allowable
max_file_uploads = 20      # per request

Note also the injection of deprecated "Pragma" caching headers by PHP, which can also be removed:

session.cache_limiter = nocache

For simple strings, debugging messages can be generated by printing to the browser with either echo or print.

For arrays, use the var_dump function.

Errors can be reported by issuing the following commands at the start of the PHP block, as suggested by this SO post:

error_reporting(-1);
ini_set('display_errors', 'On');