app_local.php

<?php
return [
    /**
     * Connection information used by the ORM to connect
     * to your application's datastores.
     * Do not use periods in database name - it may lead to error.
     * See https://github.com/cakephp/cakephp/issues/6471 for details.
     * Drivers include Mysql Postgres Sqlite Sqlserver
     * See vendor\cakephp\cakephp\src\Database\Driver for complete list
     */
    'Datasources' => [

        'default' => [
            'datasource' => 'Database/Sqlite',
            'persistent' => false,
            'database' => 'app_db',
            'prefix' => '',
            'encoding' => 'utf8',
        ],
    ]
];

bootstrap.php

<?php 
// CUTTING TOP LINES

/*
 * Read configuration file and inject configuration into various
 * CakePHP classes.
 *
 * By default there is only one configuration file. It is often a good
 * idea to create multiple configuration files, and separate the configuration
 * that changes from configuration that does not. This makes deployment simpler.
 */
try {
    Configure::config('default', new PhpConfig());
    Configure::load('app', 'default', false);
} catch (\Exception $e) {
    exit($e->getMessage() . "\n");
}

/*
 * Load an environment local configuration file.
 * You can use a file like app_local.php to provide local overrides to your
 * shared configuration.
 */
Configure::load('app_local', 'default');

error

CakePHP is NOT able to connect to the database.

Database driver Cake\Database\Driver\Mysql cannot be used due to a missing PHP extension or unmet dependency