You are supposed to edit two lines in your php.ini file (i'm using windows for this example):
-The first one is regarding the extensions directory location. See below:
; Directory in which the loadable extensions (modules) reside.
; http://php.net/extension-dir
; extension_dir = "./"
; On windows:
extension_dir = "C:/php/ext"
-The second one is regarding the extension itself:
extension=php_mysqli.dll
Only modifying (uncommenting) the extension line was not enough for me. Hope it helps