Free Software for PHP
FuelPHP
FuelPHP is a MVC (Model-View-Controller) framework that was designed from the ground up to build scalable web applications written in PHP. Out-of-the-box, FuelPHP supports HVMC as a part of its arcitecture. The Hierarchical-Model-View-Controller (HMVC) pattern is a direct extension to the MVC pattern that manages to solve many of the scalability issues when building applications. HMVC is a collection of traditional MVC triads operating as one application. Each triad is completely independent and can execute without the presence of any other. All requests made to triads must use the controller interface, never loading models or libraries outside of their own domain. FuelPHP is released as a free and open source framework under the MIT License and can be used for commercial and personal projects alike.
Magento
Magento is a free and open-source self-hosted e-commerce platform written in PHP. The software is released under the Open Software License v3.0 and has been openly developed since March 31, 2008. Today Magento powers over 200,000 online shops globally and has grown into one of the most mature and trusted open source e-commerce platform. With Magento businesses can manage their online retail business with sophisticated tools that has been tried and tested.
Guzzle
PHP HTTP client that makes it easy to send HTTP requests and trivial to integrate with web services
OpenCart
OpenCart is a free and open source e-commerce and shopping cart system written in PHP. With OpenCart merchants can create their own online store at a minimal cost, simply get a web host and start installing the software. The OpenCart software includes all the necessary features to run a online stores and connects to over 20+ payment gateways and 8 shipping methods.
osCommerce
osCommerce is an open-source shopping cart system that you can use to create online store. It is one of the oldest and most comprehensive open-source shopping cart on the market today. The software is written in PHP and is self-hosted, allowing you to have complete access to, and total control of your online store and data. osCommerce is a 20+ years old, community-driven project and includes over 9,000 add-ons that can be used to customize and add further functionality to your online store.
bbPress
bbPress is forum software from the creators of WordPress. With bbPress you can quickly setup a forum for asyncronous discussion and subscriptions for your WordPress-powered site.
Drupal Commerce
Drupal Commerce is a free and open-source eCommerce module for Drupal. With Drupal Commerce, people can start selling products from their Drupal-powered websites by simply installing and configuring the module. Drupal Commerce provides extra extensions that can be used to further improve and extend on the features of the module.
phpBB
One of the world most popular, free, open-source and self-hosted forum software, written in PHP. With phpBB you can create communities that can be hosted with most hosting providers. The software package is built and designed from the ground up to to be extensible and provides customization option through themes and feature extensions through plugins. The extensive database of user-created extensions and styles database, easy of use, and the simple installation process has made phpBB a common choice among self-hosted forums and communities.
MediaWiki
MediaWiki is a free and open source wiki engine used by thousands of organizations and websites across the globe. It's the software that powers Wikipedia, which was released as a free and open source project under the name MediaWiki, a year after the release of Wikipedia. The software allows people to collect and organize knowledge in cooperation with one another, and make it available on the web. MediaWiki is now built by a community of open source developers and provides plugins can be used to extend the software further. Allowing sister sites like Wiktionary, Wikimedia Commons and Wikidata to utilize the same code base.