Avoid hard coded databases credentials in combination with shell - Cakephp 3 - php

Question

I have written a Cakephp 3 shell application which uses models and behaviors to generate thumbnails. Since I populated the database credentials through the $_SERVER super global I need to manually insert the credentials. However, when I try the following snippet of code this results in an error saying that I can't overwrite the connection:
public main($host, $username, $password, $db){
ConnectionManager::config('default', [
'className' => 'Cake\Database\Connection',
'driver' => 'Cake\Database\Driver\Mysql',
'persistent' => false,
'host' => $host,
'username' => $username,
'password' => $secret,
'database' => $db,
'encoding' => 'utf8',
'timezone' => 'UTC',
'cacheMetadata' => true,
]);
........
}
Moreover, it first tries to load my default settings in config/app.php which aren't populated and throws notices and errors:
if (isset($_SERVER['RDS_HOSTNAME']) && isset($_SERVER['RDS_USERNAME']) && isset($_SERVER['RDS_PASSWORD']) && isset($_SERVER['RDS_DB_NAME']) ) {
define('RDS_HOSTNAME', $_SERVER['RDS_HOSTNAME']);
define('RDS_USERNAME', $_SERVER['RDS_USERNAME']);
define('RDS_PASSWORD', $_SERVER['RDS_PASSWORD']);
define('RDS_DB_NAME', $_SERVER['RDS_DB_NAME']);
}
return [
.......
'Datasources' => [
'default' => [
'className' => 'Cake\Database\Connection',
'driver' => 'Cake\Database\Driver\Mysql',
'persistent' => false,
'host' => RDS_HOSTNAME,
/**
* CakePHP will use the default DB port based on the driver selected
* MySQL on MAMP uses port 8889, MAMP users will want to uncomment
* the following line and set the port accordingly
*/
//'port' => 'non_standard_port_number',
'username' => RDS_USERNAME,
'password' => RDS_PASSWORD,
'database' => RDS_DB_NAME,
'encoding' => 'utf8',
'timezone' => 'UTC',
Is there any way to avoid hard coding of the database credentials in this way?