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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

Constant wildcards

Foo::BAR_* stands for the values of all the constants of Foo whose name starts with BAR_, and PHP_URL_* for the values of all the global constants whose name starts with PHP_URL_, built-in or defined in your code, e.g. @param PHP_URL_* $component accepts PHP_URL_SCHEME, PHP_URL_HOST and the other PHP_URL_* constants. In a namespace, an unqualified wildcard matches the constants of the namespace, or the global ones if none of the namespace match.