For an ongoing project, i need to add an extension (timezonedb) to PHP. The First thing to do, is to get the extension already built (.dll), in my case i use this link and i choose the one which match with my configuration (PHP 5.5, Thread Safety enabled, 32bits x86). You can find your configuration with phpinfo(). Once this step done, you have to paste this file in the right repertory:

C:\wamp\bin\php\php5.5.12\ext

Next, we have to add this extension to the php.ini file, which can be found in:

C:\wamp\bin\apache\apache2.4.9\bin

Example (php_timezonedb.dll):

...
;extension=php_pdo_odbc.dll
;extension=php_pdo_pgsql.dll
extension=php_pdo_sqlite.dll
;extension=php_pgsql.dll
extension=php_shmop.dll
extension=php_timezonedb.dll
...

Next start/restart Wampserver and you can see the addition of your extension with a phpinfo() or in WampServer menu (PHP > PHP extensions).