How to resolve “must be an instance of string, string given” prior to PHP 7?

Question

Here is my code:

function phpwtf(string $s) {
    echo \"$s\\n\";
}
phpwtf(\"Type hinting is da bomb\");

Which results in this error:

Answer 1

As others have already said, type hinting currently only works for object types. But I think the particular error you've triggered might be in preparation of the upcoming string type SplString.

In theory it behaves like a string, but since it is an object would pass the object type verification. Unfortunately it's not yet in PHP 5.3, might come in 5.4, so haven't tested this.

Answer 2

PHP allows "hinting" where you supply a class to specify an object. According to the PHP manual, "Type Hints can only be of the object and array (since PHP 5.1) type. Traditional type hinting with int and string isn't supported." The error is confusing because of your choice of "string" - put "myClass" in its place and the error will read differently: "Argument 1 passed to phpwtf() must be an instance of myClass, string given"

Answer 3

Prior to PHP 7 type hinting can only be used to force the types of objects and arrays. Scalar types are not type-hintable. In this case an object of the class string is expected, but you're giving it a (scalar) string. The error message may be funny, but it's not supposed to work to begin with. Given the dynamic typing system, this actually makes some sort of perverted sense.

You can only manually "type hint" scalar types:

function foo($string) {
    if (!is_string($string)) {
        trigger_error('No, you fool!');
        return;
    }
    ...
}