Before you installed the php_mysqli.dll


<?php

/* Connect to a MySQL server */
$link = mysqli_connect(
'localhost', /* The host to connect to */
'root', ...

step1. you can find php_mysqli.dll

step2. and put that dll into c:\php\extensions\

step3. that folder is not available, create it. now our dll file is in c:\php\extensions\php_mysqli.dll
...