On July 13, 2004, PHP 5 was released, powered by the new Zend Engine II.[6] PHP 5 included new features such as improved support for object-oriented programming, the PHP Data Objects (PDO) extension (which defines a lightweight and consistent interface for accessing databases), and numerous performance enhancements.[24] In 2008 PHP 5 became the only stable version under development. Late static binding had been missing from PHP and was added in version 5.3.[25][26]
Many high-profile open-source projects ceased to support PHP 4 in new code as of February 5, 2008, because of the GoPHP5 initiative,[27] provided by a consortium of PHP developers promoting the transition from PHP 4 to PHP 5.[28][29]
Over time, PHP interpreters became available on most existing 32-bit and 64-bit operating systems, either by building them from the PHP source code, or by using pre-built binaries.[30] For the PHP versions 5.3 and 5.4, the only available Microsoft Windows binary distributions were 32-bit x86 builds,[31][32] requiring Windows 32-bit compatibility mode while using Internet Information Services (IIS) on a 64-bit Windows platform. PHP version 5.5 made the 64-bit x86-64 builds available for Microsoft Windows.[33]