A colleague showed me an easy way to check, if an array contains instances of one class only:

class A {}
class B {}
class C extends A {}

$tests = [
    1 => [new A(), new A()], // succeeds
    2 => [new A(), new B()], // fails
    3 => [new C(), new C()], // succeeds, because C extends A
];

foreach ($tests as $index => $collection) {
    try {
        \array_map(static function (A $object): void {}, $collection);
    } catch (\TypeError $e) {
        // Implement your error handling here.
    }
}

Here's an example with output: https://3v4l.org/1IVg9

foreach can handle arrays and objects. You can check this with:

$can_foreach = is_array($var) || is_object($var);
if ($can_foreach) {
    foreach ($var as ...
}

You don't need to specifically check for Traversable as others have hinted it in their answers, because all objects - like all arrays - are traversable in PHP.

More technically:

foreach works with all kinds of traversables, i.e. with arrays, with plain objects (where the accessible properties are traversed) and Traversable objects (or rather objects that define the internal get_iterator handler).

(source)

Simply said in common PHP programming, whenever a variable is

• an array
• an object

and is not

• NULL
• a resource
• a scalar

you can use foreach on it.

Try to use:

if (!is_scalar($var)) {
    // Varible is object or array
}