How do I enable the mysqli extension?

On shared hosting, open the PHP version or PHP selector page in the host panel and tick mysqli (or nd_mysqli) for the version the site uses. On your own server, install the php-mysql package for that PHP version and restart PHP-FPM or the web server.

Why does the error stay after enabling mysqli?

Either WordPress is older than 3.9 and only knows PHP's removed mysql extension, or the web server runs a different PHP version than the one you checked. Check the WordPress version and a temporary phpinfo page.

More fixes

"Your PHP installation appears to be missing the MySQL extension which is required by WordPress"

Q: What does 'Your PHP installation appears to be missing the MySQL extension' mean?

The PHP version running the site has no mysqli extension enabled, so WordPress cannot talk to its database. It usually follows a PHP version change in the host panel or a server move; the files and the database are fine.

Every page of the site, the admin included, shows one plain line: "Your PHP installation appears to be missing the MySQL extension which is required by WordPress. Please check that the mysqli PHP extension is installed and enabled." Nothing is lost: the database and the files are fine. PHP simply cannot talk to the database, and that almost always started with a PHP version change.

What the owner sees

lakesideflorist.example

What the checks found

Find out which case it is (five minutes)

Ask what changed. A PHP version switch in the host panel, a host's own upgrade e-mail ("PHP 7.4 is end of life, your sites move to 8.x"), a server move or a new VPS is the cause nine times out of ten.
Open the host panel's PHP settings ("Select PHP Version", "PHP Selector", "MultiPHP Manager" or similar) and note the version the site runs now, and whether mysqli (or nd_mysqli) is ticked in the extensions list for that version.
Check how old WordPress is. Open wp-includes/version.php in the file manager and read $wp_version. Very old versions (before 3.9) only knew PHP's old mysql extension, which PHP 7 removed, so they show this message on any modern PHP even with mysqli switched on.
On your own server or VPS, check the PHP that serves the site, not the command line: they can be different versions. php -m answers for the command line only; a phpinfo() page (delete it straight after) answers for the web server.

The fixes, by case

Quickest way back up: switch the PHP version back to the one the site ran yesterday. The site returns at once, and you can then do the real fix without the shop being down.
mysqli is off for the new version (shared hosting). In the PHP extensions list, tick mysqli, or nd_mysqli on CloudLinux-style selectors (one of the two, not both), save, and reload the site. Each PHP version has its own list, so a switch can quietly reset it.
The extension is not installed (your own server). Install the MySQL package for the exact PHP version that runs the site, for example php8.2-mysql on Debian or Ubuntu, then restart PHP-FPM for that version (or the web server). Check again with a phpinfo() page and remove it.
WordPress itself is very old. Put the old PHP version back, take a full backup, update WordPress, the theme and the plugins step by step on the old PHP, and only then move to the new PHP. Expect some old plugins to need replacing; the outdated PHP guide covers the order.
Test: open the home page, log in, and add something to a cart or send the contact form. Then check the error log stays quiet for a day.

What you see, what it means, what it needs

What you see	What it means	Usual fix
Started right after a PHP version change	The new version has mysqli switched off	Tick mysqli for that version, or switch back first
Started after a server move or a new VPS	The MySQL extension was never installed there	Install the php-mysql package for that PHP version, restart PHP
mysqli is on and the message stays	WordPress older than 3.9, or the web server runs a different PHP than you checked	Check `$wp_version` and a phpinfo page
"Error establishing a database connection" instead	PHP can reach MySQL code but not the database	See the database connection guide
"There has been a critical error" after the PHP change	A plugin or theme breaks on the new PHP	See the critical error guide

What not to do

Do not edit wp-includes/load.php to remove the check. The check is right: without the extension WordPress cannot read a single post, and the edit is wiped at the next update.
Do not restore a backup or re-install WordPress. The files and the database are fine; a restore only risks losing recent orders and posts.
Do not leave a phpinfo() file on the site. It shows your server's paths and settings to anyone who finds it.
Do not keep an end-of-life PHP version for good just to make the message go away. Switching back is a way to get the site up today, not the fix.

Most of these are one tick box in the host panel and are fixed in minutes. If the site is very old, or it runs on its own server and nobody knows how it was set up, that is what the box below is for. Related: outdated PHP, database connection error and "There has been a critical error".