This page lists optional technical requirements which are not required to install and run Joomla! but are required for some internal APIs. The list was created for Joomla 4.

Item Description
Application  
JApplicationDaemon Requires PHP's ext/pcntl and ext/posix
Archive  
BZ2 Requires PHP's ext/bz2
GZip Requires PHP's ext/zlib
Zip Requires PHP's ext/zip or ext/zlib
Cache  
APC Requires PHP's ext/apc on PHP 5.3 or 5.4, ext/apcu on PHP 5.5 or 5.6, unsupported on PHP 7.x (Note: THIS NEED TO BE CHECKED)
APCu Requires PHP's ext/apcu on PHP 5.3+
CacheLite Requires the PEAR Cache_Lite package (tested on 1.7.16, will work with 1.8)
Memcache Requires PHP's ext/memcache and a Memcache server (Note: The Memcache extension is not compatible with PHP 7.x)
Memcached Requires PHP's ext/memcached and a Memcached server
Redis Requires PHP's ext/redis and a Redis server
Wincache Requires PHP's ext/wincache (Windows only)
XCache Requires PHP's ext/xcache
Client adapters  
LDAP Requires PHP's ext/ldap
HTTP/Curl Requires PHP's ext/curl
HTTP/Socket Requires PHP's fsockopen() function be enabled
HTTP/Stream Requires PHP's fopen() function and allow_url_fopen enabled
Cryptography  
JCrypt Requires PHP's ext/mcrypt for all ciphers except the SodiumCipher which requires ext/sodium
JKeychain Requires PHP's ext/openssl
Database  
Microsoft SQL Azure Requires PHP's ext/sqlsrv (the PHP 5.x extension only supports Windows, a Linux compatible version of the extension is available for PHP 7.x)
Microsoft SQL Server Requires PHP's ext/sqlsrv (the PHP 5.x extension only supports Windows, a Linux compatible version of the extension is available for PHP 7.x)
MySQL Requires PHP's ext/mysql (unsupported on PHP 7.x)
MySQLi Requires PHP's ext/mysqli
Oracle Requires PHP's ext/pdo with Oracle support (available for 3PD only; the CMS won't run with it)
PDO MySQL Requires PHP's ext/pdo with MySQL support
PostgreSQL Requires PHP's ext/pgsql
SQLite Requires PHP's ext/pdo with SQLite support (available for 3PD only; the CMS won't run with it)
Image  
  Requires PHP's ext/gd
  Requires PHP's ext/fileinfo
Session  
APC Requires PHP's ext/apc on PHP 5.3 or 5.4, ext/apcu on PHP 5.5 or 5.6, unsupported on PHP 7.x (Note: THIS NEEDS TO BE CHECKED)
Memcache Requires PHP's ext/memcache and a Memcache server (Note: The Memcache extension is not compatible with PHP 7.x)
Memcached Requires PHP's ext/memcached and a Memcached server
Wincache Requires PHP's ext/wincache (Windows only)
XCache Requires PHP's ext/xcache
OPTIONAL IMPROVEMENTS  
String  
mbstring Enable PHP's ext/mbstring for the phputf8 library to use native functions