1) in the directory where apache.exe is start from
  2) in the directory where php4apache.dll is loaded from
  3) in your %SYSTEMROOT%\System32, %SYSTEMROOT%\system and
     %SYSTEMROOT% directory.
     Note: %SYSTEMROOT%\System32 only applies to Windows NT/2000/XP)
  4) in your whole %PATH%

  Note: What is %SYSTEMROOT% ? Depending on your Windows
        installation this may be for example c:\winnt or C:\windows

  Usually you would just copy it over to %SYSTEMROOT%\System32.
  But if you want to have multiple PHP installations (for
  whatever reason) this is a bad idea. For this circumstance the
  safest thing is to let php4ts.dll reside in the same directory
  where php4apache.dll is loaded from (see point 2 above).


  After you've set up the file layout properly, you're ready to
  finally configure Apache to load the PHP4 module. Just add the
  following lines to your httpd.conf:

   LoadModule php4_module c:/php/sapi/php4apache.dll
   AddModule mod_php4.c
   AddType application/x-httpd-php .php