Mutability annotations

Psalm has few annotations for mutability. They are differ by a scope they can be applied.

Scopes

class

• @psalm-immutable: the same as readonly classes in PHP 8.2.

methods

• @psalm-mutation-free: to annotate a class method that does not mutate state (getter-like). It's not @pure as it depends on $this (class context).
• @psalm-external-mutation-free: to annotate a class method that does not mutate state externally of the class's scope.

properties

• @readonly / @psalm-readonly: to annotate a property that can only be written to in its defining class's constructor.
• @psalm-allow-private-mutation: to annotate @readonly properties that can be mutated in a private context.
• @psalm-readonly-allow-private-mutation: the same as combination of @readonly + @psalm-allow-private-mutation

functions (incl. methods)

• @psalm-pure: to annotate a pure function - one whose output is just a function of its input.