First thing, is display_startup_errors set to On in php.ini ?
If you do a phpinfo(), does it show the correct path to your php.ini file as well as extension_dir ?
Have you added the PHP install directory to the system PATH environment variable? Have you copied the files in PHP_INSTALL_DIR\dlls to PHP_INSTALL_DIR\ (substitue PHP_INSTALL_DIR for wherever you decided to install PHP) ?
Lastly, what version of PHP are you running?