src/StreamWrapper.php
public static function emulate($emulation, callable $assertionCallable = null)
    {
        if ((is_string($emulation)) && (class_exists($emulation))) {
            $emulation = new $emulation($assertionCallable);
        }

        static::$emulation = $emulation;
    }
public static function emulate($emulation, callable $assertionCallable = null)
    {
        if ((is_string($emulation)) && (class_exists($emulation))) {
            $emulation = new $emulation($assertionCallable);
        }

        static::$emulation = $emulation;
    }
Set the emulation to use.

@param string|callable|object $emulation The emulation to use.
@param callable|null $assertionCallable The assertion call to use.
@return void