Dev-PHP is a lightweight development environment for PHP. Dev-PHP is hardly comparable with the likes of Zend Studio or Nusphere but is more than a syntax editor.

It provides syntax highlighting for CSS, JavaScript, HTML, XML and even SQL. Currently in alpha release, Dev-PHP also supports development of PHP-GTK applications, an internal web browser, class browser and convenient access to your PHP documentation.

You can use it to create scripts and programs using the PHP scripting language and the PHP-GTK library.
Dev-PHP is free software under the GNU General Public License, which means you can freely modify or use the source code under the terms of the license.

Link to the official website: http://devphp.sourceforge.net