Which version of PHP are you using and what method did you use to install it? The version of the .dll must be the same as the .h header files that were used when PHP was built.
For PHP 4 have you checked the "extensions" folder or for PHP 5 the "ext" folder?
What is the setting for "extension_dir =" in your php.ini file?
Also, if the following line in your php,ini file is commented out (semi-colon
; at the start of the line), you must remove the
; and then stop and start your web server to get the change to take effect -