I have been using netbeans for several years.
it can be the best for beginner with features like syntax highlighting, error highlighting, auto completion and few more. and can be best for professional programmers when you need support for CVS versioning, cake php, smarty framework, symphony framework and a very customizable and accessible user interface and several other plugins can be installed. it is free and open source and available for windows, Linux, mac and computer which have a java run time environment(JRE) or java development Kit(JDK).
i have attached a screenshot of netbeans 7.1 under a windows operating system.
just download the jdk from http://www.java.com
and netbeans from http://www.netbeans.org
and test it.
And windows user can also use a unique tool from Microsoft for free. Microsoft web matrix.
the beta2 version of Microsoft offers code auto completion for php, css, javascript, html and most of other softwares like joomla, wordpress, syntax highlighting, ftp (editing remote sites), web deployment, error highlighting, an office 2010 like interface and some others