Question

Why is it possible to create an interface without specifying a return type? Why doesn’t this make this interface unusable?

This makes it more clear:

Interface run
{
    public function getInteger();
}

class MyString implements run
{    
    public function myNumber()
    {

    }

    public function getInteger()
    {  
        return "Not a number";
    }    
}

In Java every Interface has a return type like Integer, String or Void

I know that PHP is unfortunately a loosely typed language but isn’t there a solution to that problem?

Is it possible to define an Interface with a return type like Integer?

Answer 1

Type hinting for function/method arguments or return values is only supported at PHP7.0 or later

Check the details here:
http://php.net/manual/en/migration70.new-features.php

If you are using PHP5 so the current accepted practice is to use phpdoc comments to indicate the “contract” is present.

/** 
 * Does something.
 * @return int
 **/
public function getInteger() { return 1; }

If the code violates the “contract,” I suggest finding the original coder and having them fix it and/or filing a bug and/or fixing it yourself if it’s in your own codebase.