A small compendium of what is possible with `vimeo/psalm` 3.9.x to add some decent type system features to PHP

psalm-compendium.php

<?php

// -- types are a compile-time propagated concept
// https://psalm.dev/r/338f74a96c
class TheType
{
    /** @var string */
    public $foo = 'bar';
}

/** @param TheType $typed */
function doSomethingWith($typed) : void
{
    // While PHP supports types in method signatures, this is sufficient,
    // when using a type checker
    $typed->foo;
}












// -- constant types
// https://psalm.dev/r/ae1aadffc7
class SomeConstants
{
    const C = 'c';
    const OTHER_D = 'd';
    const OTHER_E = 'e';
}

/** @psalm-param 1|2|'a'|'b'|SomeConstants::C|SomeConstants::OTHER_* $parameter */
function intOrAOrB($parameter): void
{
    echo $parameter;
}

intOrAOrB(1);
intOrAOrB('b');
intOrAOrB('c');
intOrAOrB('d');
intOrAOrB('B'); // error








// -- true and false types
// https://psalm.dev/r/1d1c889ade
/** @psalm-return true */
function alwaysTrue(int $input) : bool
{
    return (bool) $input; // error
}





















// -- array types
// https://psalm.dev/r/16a451edf6
/**
 * @psalm-param array{
 *  key: string,
 *  optionalKey?: string,
 *  nullableKey: ?string
 * } $parameter
 */
function requiresSpecificArrayShape(array $parameter) : void
{
    echo $parameter['key'];
}















// -- lists
// https://psalm.dev/r/c5de1156b5
/**
 * @psalm-param list<string> $parameter
 *
 * @psalm-return list<int>
 */
function requiresList(array $parameter) : array
{
    return array_keys($parameter);
}

















// -- non-empty lists
// https://psalm.dev/r/ab304679a9
/** @psalm-param list<string> $parameter */
function requiresPossiblyEmptyList(array $parameter) : string
{
    return $parameter[0]; // error
}

/** @psalm-param non-empty-list<string> $parameter */
function requiresNonEmptyList(array $parameter) : string
{
    return $parameter[0];
}















// -- intersection and union types
// https://psalm.dev/r/2323ab1156
interface HasA { function doA() : void; }
interface HasB { function doB() : void; }

/** @psalm-param HasA&HasB $ab */
function doAAndB($ab): void
{
    $ab->doA();
    $ab->doB();
}

/** @psalm-param HasA|HasB $ab */
function doAOrB($ab): void
{
    $ab->doA(); // error
    $ab->doB(); // error

    assert(! $ab instanceof HasB);

    $ab->doA(); // OK
}







// -- templated types
// https://psalm.dev/r/c628b74ea1
class Thing { function doTheThing(): void { echo "ok"; } }
/** @psalm-template ContainedThing as mixed */
interface ContainsSomething
{
    /** @psalm-return ContainedThing */
    function get();

    /**
     * @psalm-template ThingToBeWrapped as mixed
     * @psalm-param ThingToBeWrapped $thing
     * @psalm-return self<ThingToBeWrapped>
     */
    public static function make($thing);
}

/** @psalm-param ContainsSomething<Thing> $container */
function doTheThing($container): void
{
    $container->get()->doTheThing(); // OK
}

/** @psalm-param class-string<ContainsSomething> $containerClass */
function makeAndDoTheThing(string $containerClass): void
{
    $containerClass::make(new Thing())->get()->doTheThing();
}



// -- iterable is a generic type
// https://psalm.dev/r/07eafb40ae
interface ItemType { function doSomething(int $index): void; }

/** @psalm-return iterable<int, ItemType> */
function makeIteratorOfItemType(): iterable
{
    return [];
}

foreach (makeIteratorOfItemType() as $key => $item) {
    $item->doSomething($key);
}















// -- read-only
// https://psalm.dev/r/3e8db86f49
class HasReadOnlyProperties
{
    /**
     * @var string
     * @psalm-readonly
     */
    public $foo = 'a';
}

$hasReadOnlyProperties = new HasReadOnlyProperties();
$hasReadOnlyProperties->foo = 'b'; // error















// -- mutation-free
// https://psalm.dev/r/4f5e2815e9
function unsafeFunction() : void { /* does nothing, but we don't know */ }

final class MethodDoesNotMutate {
    /** @var int */
    private $state = 0;

    /** @psalm-mutation-free */
    function cannotMutate() : string {
        $this->state++; // error

        return 'ha';
    }

    /** @psalm-external-mutation-free */
    function cannotMutateOthers(self $others) : string {
        $this->state++; // OK
        unsafeFunction(); // error

        return 'ha';
    }
}






// -- templated type inheritance
// https://psalm.dev/r/692afc91cf
class Goodies {}
/** @psalm-template ItemType */
interface MyCollection {
    /** @psalm-return iterable<ItemType> */
    function get();
}

/** @extends MyCollection<Goodies> */
interface CollectionOfGoodies extends MyCollection{

}

/** @implements MyCollection<Goodies> */
class ConcreteCollectionOfGoodies implements MyCollection {
    function get() { return []; }
}










// -- assertions
// https://psalm.dev/r/4c3aaf5e82
/**
 * @psalm-assert !'a' $value
 * @psalm-assert !int $value
 * @param mixed $value
 */
function assertItIsNotThatThing($value): void
{
    // @TODO exceptions here
    echo gettype($value);
}

/**
 * @psalm-param 'a'|'b'|'c'|1|2 $value
 * @psalm-return 'b'|'c'
 */
function useThePreciseType($value): string
{
    assertItIsNotThatThing($value);

    return $value;
}







// -- supporting both psalm and the IDE or other tools
// https://psalm.dev/r/115e4f1814
/**
 * @psalm-template TKey
 * @psalm-template TValue
 */
interface Collection {}
/**
 * @psalm-template TKey
 * @psalm-template TValue
 * @implements Collection<TKey, TValue>
 */
final class ArrayCollection implements Collection {}
class ReferencedEntity {}
class MyEntity {
    /**
     * @var Collection|ReferencedEntity[]
     * @psalm-var Collection<int, ReferencedEntity>
     */
    private $collection;
    public function __construct() {
        $this->collection = new ArrayCollection();
    }
}

Examples for templated callables for properly typed higher-order functions: https://gist.github.com/bcremer/2d056761019c5119328ff33cefb157db