The Zend Engine powers the standard PHP interpreter, and is a free software released under the PHP License. PHP has been widely ported and can be installed on most web servers on almost every OS, and platform, free of charge.
The PHP language evolved without a written formal specifications or standards until 2014, leaving the canonical PHP interpreter as a de facto standard. Work has been ongoing to create a formal PHP specification.
On 07 Jan 2016 the PHP development team has recently announced the immediate availability of PHP 5.6.17. This is a security release. Many security bugs were fixed in this release. PHP 5.6 or later users are encouraged to upgrade to this version.