How to rename multiple files in multiple folders with one command

get-childItem -recurse | Where {$_.extension -eq ".html"} | rename-item -newname { $_.name -replace ".html",".php" }

This will work in PowerShell.  If you have Windows 7 or Vista, you should have it installed by default.  If you are on XP you can download it here.

In command line:

for /f "delims=*" %a in ('dir *.html /b /s') do ren "%a" *.php

Note: You can replace *.html for other wildcard, e.g. d:\www\*.html.

Note 2: If using the command within a batch file, replace %a with %%a (don't ask me why)

Using forfiles, we can write a script, to rename files recursively in all subfolders.

forfiles /S /M *.html /C "cmd /c rename @file @fname.php"   

Source: Batch script for renaming files in bulk