Which tool is mainly used for developing Web application in PHP?

Basic tools for web development are as follows:

1. A browser - FF/Chrome/Safari to name a few so that the codes you are doing are seen visually.

2. A Web/Database/NoSQL Server - XAMPP, MAMP, IIS or any web/application server that fits your need... Apache, Nginx, LightHTTPD, LiteSpeed, Cherokee, Tomcat, MySQL, Redis etc.

3. An Editor - IDE or plain text will do like notepad, notepad++, Zend Studio, Sublime Text3 etc.

4. Debugging Tools - Firebug, Live Headers, XDebug etc...

I guess that's the basic one that is on top of my head...

There are several tools. The best paid tools are Zend studio and Adobe Dreamweaver . 

Free tools Aptana studio, eclipse and netbeans with php plugins. 

There are also a few light weight editors like note pad++ which are not that rich in features.

If you are asking for IDE here are the options.

- Zend Studio (paid, free for Zend Certified developers)

- Eclipse (free)

- Netbeans (free)

If you are a plain texteditor lover then go for Sublime Editor.