SDL_GetKeyboardState Argument #1 ($numkeys) cannot be passed by reference

$keyboard = SDL_GetKeyboardState($var = null);

if($keyboard(SDL_SCANCODE_LEFT)) {
	print('left');
	$bar_x += -1;
}
		
if($keyboard(SDL_SCANCODE_RIGHT)) {
	print('left');
	$bar_x += +1;
}