PHP (recursive acronym for: PHP Hypertext Preprocessor), is a widely-used, Open Source embedded scripting language. PHP is a server-side scripting language usually written in an HTML context. Unlike an ordinary HTML page, a PHP script is not executed on client browser; instead, it is parsed by the server then sends output to client browser.

A PHP script code can query databases, create images, embed other PHP file, read and write files — the possibilities and functionalities of PHP are endless.

If you are a PHP programmer then you might be looking an IDE which supports syntax highlighting for easy code recognition, WYSIWYG environment, auto-code completion (just type few characters and it will show you the relevant tags), multiple language support along with PHP (like HTML, SQL, JavaScript, CSS, XML etc) so that you don’t need to switch from one editor to another. Not only this, you might also want such feature rich tools for free.

I am going to lists some of the best and freely available editors for PHP. Most of them are free to download and distribute under GNU/GPL license. The following are some of the best free editors for PHP web-application development.

NOTE : Before you download any free WYSIWYG PHP editors please read this carefully. Most of these editors require you to download PHP, MySQL, and Apache and configure them properly to run PHP locally. For different platforms (Operating System) different types of software available, so to run them successfully you need to configure them properly. The easiest way to run PHP is to download preconfigured software which includes PHP, Apache server, MySQL and extra features.

WAMP (Windows, Apache server, MySQL and PHP): Windows only.
LAMP (Linux, Apache server, MySQL and PHP) : Linux Only (No one click installer available).
MAMP (MAC, Apache, MySQL and PHP) : Mac OS only.
XXAMP (An OS, Apache, MySQL and PHP) : Available for Linux, MAC, Windows and Solaris.

Notepad++:

Notepad++ is a free (as in "free speech" and also as in "free beer") open source code editor and Notepad replacement that supports several languages. Running in the MS Windows environment, its use is governed by GPL License.

Features of Notepad++:

• 40+ programming Language support.
• Syntax Highlighting and Syntax Folding
• PHP WYSIWYG editor
• User Defined Syntax Highlighting
• Auto-completion
• Multi-Document
• Multi-View
• Regular Expression Search/Replace supported
• Full Drag ‘N' Drop supported
• Dynamic position of Views
• File Status Auto-detection
• Zoom in and zoom out
• Multi-Language environment supported
• Bookmark
• Brace and Indent guideline Highlighting
• Macro recording and playback
• Free plug-in support
• Custom theme support etc.

License: Open source, GPL.
Platform: Windows.
Download Page

Netbeans:

The NetBeans IDE is a free, open-source Integrated Development Environment (IDE) for software developers. You get all the tools you need to create professional desktop, enterprise, web and mobile applications, in Java, C/C++ and a variety of dynamic languages. The IDE runs on many platforms including Windows, Linux, Solaris, and Mac OS; it is easy to install and use straight out of the box.

NetBeans Supported Technologies
Ajax | C/C++ | Databases | Debugger | Desktop | Editor | Groovy | GUI Builder | Java EE | JavaFX | Java ME | Java SE | JavaScript | Mobile | PHP | Profiler | Python | Refactor | REST | Rich Client Platform | Ruby | SOA | SOAP | UML | Web | WSDL | XML

NetBeans General IDE features:

• Non-proprietary Apache Ant 1.7.1 build scripts
• Projects, Files, Services, and Favorites views
• Fully configurable user interface (fonts, colors, toolbars, views)
• Code formatting settings customizable per project
• All views can be positioned, maximized/minimized, split, cloned and (un)docked
• Navigator with members and hierarchy inspectors
• Open As... action for associating file extensions with MIME-types
• Project grouping
• Included templates and sample applications
• Go to File, Go to Type, Go to Symbol, Go to Declaration actions
• Head-less build support
• Task List integration (iCalendar format): Automatic scanning for errors, warnings, TODO/FIXME tasks, and bug database integration
• Sharable projects (relative library paths, name variable paths), support for MS Windows UNC paths.
• Eclipse project and JBuilder project importer, Eclipse project synchronization
• Quick Search bar: Quick access to files/types/symbols, projects, option panels, menu actions, and documentation

There is a huge list of its feature, technologies and languages it supports. I couldn’t include all features because only NetBeans feature will take a whole page to describe. If you work on multiple programming languages then you must download and install this great tool. It includes PHP database tools, open source PHP editor, JavaScript editor, CSS editor etc.

Licence: GNU Lesser General Public License version 2.1 (LGPL)/GNU General Public License version 2 (GPL)
Platform: Mac OS X, Linux, Solaris and Windows.
Download Page

Komodo Edit:

Komodo Edit is a free, multi-language PHP source editor with code intelligence. Komodo IDE builds on Komodo Edit's comprehensive editing features, adding intelligent tools for debugging, regular expressions, team development and customization.

Features of Komodo Edit:

• Multi programming language support.
• Code Folding
• WYSIWYG editor
• Syntax Highlighting and Coloring
• Syntax checking
• Auto-completion
• Multi-Document editing
• Multi-View
• HTML preview
• Multi-Language environment supported
• XPI Extensions
• Project manager etc.

License: Mozilla Public License (MPL), GNU General Public License (GPL), and GNU Lesser Public License (LGPL).
Platform: Linux, Mac OSX and Windows.
Download page

Aptana Studio:

Aptana PHP is the robust, free, open source IDE for PHP featuring everything you need to get started fast and get even faster with developing, testing, tuning, and deploying your PHP apps. From pre-installed PHP servers, code assist, code templates, code generation, debugging, refactoring, database tools and more, Aptana PHP gives you the end-to-end tools you need for PHP plus all the rest that Aptana Studio has to offer.

• Integrated Code Assist
• Debugging
• Code Colorization
• Refactoring
• Outline
• Jump To Declaration
• PHPDoc Generation
• TODO Tasks
• Remote Debugging (Beta)
• Based on Eclipse (Supports all Eclipse plug-ins) 

License: NA.
Platform: Linux, Mac OSX and Windows.
Download Page

Eclipse PDT (PHP Development Tools Project):

This free IDE Framework supports many languages with code formatting, debugging and parsing. You will need to download PHP plug-in PHPEclipse from sourceforge.net.

The PDT project provides a PHP Development Tools framework for the Eclipse platform. This project encompasses all development components necessary to develop PHP and facilitate extensibility. It leverages the existing Web Tools Platform (WTP) and Dynamic Languages Toolkit (DLTK) in providing developers with PHP capabilities. Read FAQ before installing PDT.

License: Eclipse Public License (EPL) and others.
Platform: Linux, Mac OSX and Windows.
Download page

PSPad: a freeware code editor:

PSPad is a completely free multi programming language programmer's editor for Microsoft Windows Systems.

PSPad has following features:

• Work with projects
• Work with several documents at the same time (MDI)
• Save desktop sessions to later reopen all session files
• FTP client - edit files directly from the web
• Macro recorder to record, save and load macros
• Search and replace in files
• Text difference with color-coded differences highlighted
• Templates (HTML tags, scripts, code templates...)
• Installation contains templates for HTML, PHP, Pascal, JScript, VBScript, MySQL, MS-Dos, Perl,
• syntax highlighting according to file type
• User-defined highlighters for exotic environments
• Auto correction
• Intelligent internal HTML preview using IE and Mozilla
• Full HEX editor
• Call different external programs for different environments
• External compiler with output catcher, log window and log parser for an "IDE" effect in every Environment
• Color syntax highlight printing and print preview
• Integrated TiDy library for formatting and checking HTML code, conversion to CSS, XML, XHTML
• Integrated free version of the top CSS editor TopStyle Lite
• Export with highlight to RTF, HTML, TeX format to file or clipboard
• Column block select, bookmarks, line numbers, ...
• Reformat and compress HTML code, tag character case change
• Line sorting with ability to sort on defined columns and drop duplicates
• ASCII chart with HTML entities
• Code explorer for Pascal, C/C++, INI, HTML, XML, PHP and more in development
• Spell checker
• Internal web browser with APACHE support
• Matching bracket highlighting
• ...(many more features, too numerous to list)

License: NA.
Platform: Windows.
Download Page

BlueFish

Bluefish is a powerful editor targeted towards programmers and web designers, with many options to write websites, scripts and programming code. Bluefish supports many programming and markup languages, and it focuses on editing dynamic and interactive websites.

For:

• Multiple format support like C/C++, ChangeLogs, CSS, CFML, D, gettext PO, HTML, Java, JavaScript, JSP, Octave/MATLAB, Pascal, Perl, PHP, Python, R, Ruby, Shell, SQL, Tcl, XML.
• Multiple document interface, easily opens 500+ documents (tested 3500 documents simultaneously).
• Support for remote files using gnome-vfs (FTP, SFTP, HTTP, HTTPS, WebDAV, Samba and more).

Platform: UNIX, Linux, Mac OSX.
License: Open source.
Download Page

GNU emacs

GNU Emacs is an extensible, customizable text editor—and more.

• Content-sensitive editing modes, including syntax coloring, for a wide variety of file types including plain text, source code, and HTML.
• Complete built-in documentation, including a tutorial for new users.
• Support for many languages and their scripts, including all the European “Latin” scripts, Russian, Greek, Japanese, Chinese, Korean, Thai, Vietnamese, Lao, Ethiopian, and some Indian scripts.

Platform: GNU/Linux, FreeBSD, AIX 4.3.3 and higher, Mac OS X, MS DOS, MS Windows, NetBSD, OpenBSD, Solaris, SunOS, Ultrix.
License: Open Source, GPL
Download Page

Geany:

Geany is a small and lightweight Integrated Development Environment. It was developed to provide a small and fast IDE, which has only a few dependencies from other packages. Another goal was to be as independent as possible from a special Desktop Environment like KDE or GNOME - Geany only requires the GTK2 runtime libraries.

Some basic features of Geany:

• Syntax highlighting
• Code folding
• Symbol name auto-completion
• Construct completion/snippets
• Auto-closing of XML and HTML tags
• Call tips
• Many supported file types including C, Java, PHP, HTML, Python, Perl, Pascal (full list)
• Symbol lists
• Code navigation
• Build system to compile and execute your code
• Simple project management
• Plug-in interface (see Plug-ins)

Platform: Linux, FreeBSD, NetBSD, OpenBSD, MacOS X, AIX v5.3, Solaris Express and Windows.
License: GNU General Public Licence.
Download Page

Dev-PHP IDE:

Dev-PHP is a well-featured integrated development environment (IDE). You'll be able to create scripts and applications using the PHP scripting language and the PHP-GTK library (both included in the "PHP Package").

Platform: Linux, FreeBSD, NetBSD, OpenBSD, MacOS X, AIX v5.3, Solaris Express and Windows.
License: GNU General Public Licence.
Download Page

PHPEclipse :

A PHP IDE built as an Eclipse plug-in. It takes advantage of a well-designed, robust and widely used application framework.

Features: Syntax Coloring, Brackets Matching, Code folding, Code Completion, Parameter hints, Mark Occurrences, Hover Tooltips, PHP Manual integration, Code templates 3rd Party plug-in support  are few features of PHPEclipse.

License: Open source, Common Public License
Platform: Linux, Solaris, Mac OSX and Windows.
Download Page

jEdit

jEdit is a mature programmer's text editor with hundreds (counting the time developing plug-ins) of person-years of development behind it. It supports more than 130 file types.

Features:

• Written in Java, so it runs on Mac OS X, OS/2, UNIX, VMS and Windows.
• Built-in macro language; extensible plug-in architecture. Dozens of macros and plug-ins available.
• Plug-ins can be downloaded and installed from within jEdit using the "plug-in manager" feature.
• Folding for selectively hiding regions of text.
• Multiple language syntax highlighting.
• Highly configurable and customizable.
• The jEdit core has a large collection of plug-ins.

Licence: GPL 2.0
Platform: Mac OS X, OS/2, UNIX, VMS and Windows.
Download Page

These are the few tools which I have found it useful for all new and professional PHP developers. If you think I have not included any useful PHP editor then feel free to tell me and I will add that as soon as possible in this list. Thank you for reading me.