Fatal error with mysql_connect after upgrading to PHP 7

When you update your WordPress site under PHP 7, the following errors may appear :

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in ...wp-includes/wp-db.php:1665
Stack trace: #0 ...wp-includes/wp-db.php(632): wpdb->db_connect() #1 ...wp-includes/load.php(425)
Fatal error: Uncaught Error: Call to undefined function wp_kses_normalize_entities() in .../wp-includes/formatting.php:4316
Stack trace: #0 .../wp-includes/class-wp-fatal-error-handler.php(190)

By upgrading to PHP 7.3 (or rather from PHP 7.0.0 onwards), the mysql_connect extension has been removed. It was already deprecated since PHP 5.5.0.

Instead, it is recommended to use the mysqli extensions to connect to a database. This extension is more powerful and more secure.

Depending on the origin of the error, there are several solutions:

• Contact your web host and ask if mysqli extensions are enabled.
• Check that you have mysqi enabled on your hosting:
  $var_mysqli = function_exists('mysqli_connect'); var_dump($var_mysqli);
• Test your connection with mysqli :
  $mysqli_connection = new MySQLi('nom_d_hôte', 'nom_utilisateur', 'mot_de_passe', 'nom_de_la_base_de_données');
• Check your wp-config.php file and verify that the following variable :
  define(‘WP_USE_EXT_MYSQL’, false);
  is set to “false”.