PHP is the most popular, open source, server side scripting language in the world. It is the driving force behind some of the biggest website (like Facebook, Wikipedia, Yahoo, Flickr etc.) on internet.
PHP has multitude of development frameworks like CakePHP and CodeIgniter; CMS and ecommerce solutions like Magento, Joomla, Drupal which can be integrated with databases like MySQL, SQL etc.
StakLiA Solutions has good experience working on custom PHP websites and applications.