Laravel – Using $this when not in object context

I am working on a PHP library in Laravel. I want to call a variable that is global to my function and I get this error.

Using $this when not in object context

Where am I missing?

class ConfigurationProvider implements ConfigurationProvider
{
    protected $conf;

    private function getCredentials()
    {
        return $this->conf;
    }

}

My Controller

public function testApi(){
        $object = ConfigurationProvider::getCredentials();
        return $object;
}

>Solution :

You’re treating getCredentials() as a static method. Static method do not have access to the object. Just static properties.

class ConfigurationProvider implements ConfigurationProvider
{
    protected static $conf;

    private static function getCredentials()
    {
        return self::$conf;
    }
}

$object = ConfigurationProvider::getCredentials();