To install the mysqli extension for PHP, use the --with-mysqli=mysql_config_path/mysql_config configuration option where mysql_config_path represents the location of the mysql_config program that comes with MySQL versions greater than 4.1.
With versions of PHP 5.3 and newer, you can alternatively use --with-mysqli=mysqlnd. This is the recommended option as using the MySQL Native Driver results in improved performance and gives access to features not available when using the MySQL Client Library. Refer to What is PHP's MySQL Native Driver? for a brief overview of the advantages of MySQL Native Driver.
If you would like to install the mysql extension along with the mysqli extension you have to use the same client library to avoid any conflicts.
The mysqli extension is not enabled by default, so the php_mysqli.dll DLL must be enabled inside of php.ini. Also, PHP needs access to the MySQL client library. A file named libmysql.dll is included in the Windows PHP distribution and in order for PHP to talk to MySQL this file needs to be available to the Windows systems PATH. See the FAQ titled "How do I add my PHP directory to the PATH on Windows" for information on how to do this. Although copying libmysql.dll to the Windows system directory also works (because the system directory is by default in the system's PATH), it's not recommended.
On Windows, for PHP versions 5.3 and newer, the mysqli extension uses the MySQL Native Driver by default. This means you don't need to worry about configuring access to libmysql.dll.
As with enabling any PHP extension (such as php_mysqli.dll), the PHP directive extension_dir should be set to the directory where the PHP extensions are located. See also the Manual Windows Installation Instructions. An example extension_dir value for PHP 5 is c:\php\ext
Note: If when starting the web server an error similar to the following occurs: "Unable to load dynamic library './php_mysqli.dll'", this is because php_mysqli.dll and/or libmysql.dll cannot be found by the system.