generic-variance.php

<?php

class Source<out T> {
  public static function returnOne(): T { ... }
}
class Sink<in T> {
  public static function consumeOne(T $item): void { ... }
}
  
class Foo {}
class SpecialFoo extends Foo {}
    
var_dump(new Source<Foo> instanceof Source<Foo>); // bool(true)
var_dump(new Source<SpecialFoo> instanceof Source<Foo>); // bool(true): returning a SpecialFoo means you return a Foo
var_dump(new Source<Foo> instanceof Source<SpecialFoo>); // bool(false): class may return a Foo which is not a SpecialFoo
  
var_dump(new Sink<Foo> instanceof Sink<Foo>); // bool(true)
var_dump(new Sink<SpecialFoo> instanceof Sink<Foo>); // bool(false): class will refuse a Foo which is not a SpecialFoo
var_dump(new Sink<Foo> instanceof Sink<SpecialFoo>); // bool(true): class will accept any SpecialFoo

multiple-parameters.php

<?php

class Mapper<in SOURCETYPE, out TARGETTYPE> {
   public static function map(SOURCETYPE $input): TARGETTYPE { ... }
}

class Foo {}
class SpecialFoo extends Foo {}

var_dump(new Mapper<Foo, Foo> instanceof Mapper<Foo, Foo>); // bool(true)
var_dump(new Mapper<Foo, Foo> instanceof Mapper<Foo, SpecialFoo>); // bool(false) incompatible TARGETTYPE
var_dump(new Mapper<Foo, Foo> instanceof Mapper<SpecialFoo, Foo>); // bool(true)
var_dump(new Mapper<Foo, Foo> instanceof Mapper<SpecialFoo, SpecialFoo>);  // bool(false) incompatible TARGETTYPE 

var_dump(new Mapper<Foo, SpecialFoo> instanceof Mapper<Foo, Foo>); // bool(true)
var_dump(new Mapper<Foo, SpecialFoo> instanceof Mapper<Foo, SpecialFoo>); // bool(true)
var_dump(new Mapper<Foo, SpecialFoo> instanceof Mapper<SpecialFoo, Foo>); // bool(true)
var_dump(new Mapper<Foo, SpecialFoo> instanceof Mapper<SpecialFoo, SpecialFoo>); // bool(true)

var_dump(new Mapper<SpecialFoo, Foo> instanceof Mapper<Foo, Foo>); // bool(false) incompatbile SOURCETYPE
var_dump(new Mapper<SpecialFoo, Foo> instanceof Mapper<Foo, SpecialFoo>); // bool(false) incompatible TARGETTYPE
var_dump(new Mapper<SpecialFoo, Foo> instanceof Mapper<SpecialFoo, Foo>); // bool(true)
var_dump(new Mapper<SpecialFoo, Foo> instanceof Mapper<SpecialFoo, SpecialFoo>); // bool(false) incompatible TARGETTYPE

var_dump(new Mapper<SpecialFoo, SpecialFoo> instanceof Mapper<Foo, Foo>); // bool(false) incompatbile SOURCETYPE
var_dump(new Mapper<SpecialFoo, SpecialFoo> instanceof Mapper<Foo, SpecialFoo>); // bool(false) incompatbile SOURCETYPE
var_dump(new Mapper<SpecialFoo, SpecialFoo> instanceof Mapper<SpecialFoo, Foo>); // bool(true)
var_dump(new Mapper<SpecialFoo, SpecialFoo> instanceof Mapper<SpecialFoo, SpecialFoo>); // bool(true)