Hello,
I have a VPS with CentOS 7. I have all LAMP installed, but not mysqli (it's not showing under php -m | grep -i mysql). My php version is 5.5.36. If I do
I get the error:
How do I install the extension?
Thanks.

That means it's not loaded but it may be installed in your system

Check in extension directory
php -i | grep extension_dir

Edit: maybe the package is php-mysql, not php-mysqli
http://php.net/manual/en/mysqli.installation.php

Anyway not expert in Centos here, but web search engine is always useful

Thanks for the reply.
php -i | grep extension_dir
outputs:
extension_dir => /usr/local/lib/php/extensions/no-debug-non-zts-20121212 => /usr/local/lib/php/extensions/no-debug-non-zts-20121212
That path is a directory that contains only three files: pdo.so pdo_mysql.so pdo_sqlite.so
What do I do with them?
Maybe I have mysqli installed as you say, but I don't know how to enable it. I can't find a reference that gives clear instructions on it. Can you help me on that?
Thanks.

mysqli should be included as part of the php-mysql package. There should be a mysqli.ini file in /etc/php.d/ which enables the extension in php.

Make a php page to run phpinfo() which will tell you for sure if php recognizes mysqli.

Also, centos7 installs mariadb as a plug-in replacement for mysql.

Doug G, I didn't try your solution because I found one on the web that worked:

To install mysqli using EachApache:

1. Login to WHM as 'root' user.
2. Either search for "EasyApache" or go to Software > EasyApache
3. Scroll down and select a build option (Previously Saved Config)
4. Click Start "Start customizing based on profile"
5. Select the version of Apache and click "Next Step".
6. Select the version of PHP and click "Next Step".
7. Chose additional options within the "Short Options List"
8. Select "Exhaustive Options List" and look for "MySQL Improved extension"
9. Click "Save and Build"