php - Fatal error: Can't use function return value

empty requires a variable as if the parameter were passed by reference:

Note:
empty() only checks variables as anything else will result in a parse error. In other words, the following will not work: empty(trim($name)).

So you could do this:

$var = is_resource(true);
echo empty($var);

But as is_resource already returns a boolean value, you actually don’t need another testing function.