Uncaught Error: Call to undefined function mysql_connect() – WordPress installation

As Pat J says, you shouldn’t have define( ‘WP_USE_EXT_MYSQL’, true); unless you need the old mysql functions, which you do not if you’re using PHP 7x and WordPress 5x. You do need to restart Apache after modifying php.ini. What do you have in your mysqli section of your phpinfo? Make sure extension_dir is set in php.ini, too. You may need to enable php.ini error logging to see if your mysqli extension is loading by setting display_startup_errors = On
set error_log to [filename] or syslog.

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