gistfile1.txt

<?php


/// a built-in interface
interface TargetAttribute {
    
    public function validate($target): boolean;
}


interface NonFooInterface {
}

<<PhpAttribute>>
class SingleArgument implements TargetAttribute {

    public function __construct($argumentValue) {
    }
        
    public function validate(NonFooInterface $target): boolean {
        return true; // you could add some complex logic here as well. 
    }
}


<<SingleArgument>> /// this would fail since the target, in this case Foo, doesn't implement the NonFooInterface
class Foo {
    
    public __construct(string <<SingleArgument>> $prop) // this would also fail because it passes a string.
}