Installing the PHPEclipse plugin is quite straightforward :

- Make sure you have Eclipse 2.0 properly setup or download it from www.eclipse.org 

- Simply unzip the files in your eclipse plugins 
  directory (<your path>\eclipse\plugins). 

- (re)Start Eclipse. 

- Open the "Resource Perspective" in the Eclipse IDE and add a file with a 
  *.php, *.php3 or *.php4 extension to your source tree. 
  With a right mouse click on the PHP file you should see the new menu entry 
  "Open With->PHP Editor". 

- For more Information go to the menu "Help->Help Contents" and 
  open the "PHPEclipse Help"