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Value types

Psalm also allows you to specify values in types.

null

This is the null value, destroyer of worlds. Use it sparingly. Psalm supports you writing ?Foo to mean null|Foo.

true, false

Use of true and false is also PHPDoc-compatible

"some_string", 4, 3.14

Psalm also allows you specify literal values in types, e.g. @return "good"|"bad"

Regular class constants

Psalm allows you to include class constants in types, e.g. @return Foo::GOOD|Foo::BAD. You can also specify explicit class strings e.g. Foo::class|Bar::class

If you want to specify that a parameter should only take class strings that are, or extend, a given class, you can use the annotation @param class-string<Foo> $foo_class. If you only want the param to accept that exact class string, you can use the annotation Foo::class:

<?php
class A {}
class AChild extends A {}
class B {}
class BChild extends B {}

/**
 * @param class-string<A>|class-string<B> $s
 */
function foo(string $s) : void {}

/**
 * @param A::class|B::class $s
 */
function bar(string $s) : void {}

foo(A::class); // works
foo(AChild::class); // works
foo(B::class); // works
foo(BChild::class); // works
bar(A::class); // works
bar(AChild::class); // fails
bar(B::class); // works
bar(BChild::class); // fails