Top types, bottom types


This is the top type in PHP's type system, and represents a lack of type information. Psalm warns about mixed types when the reportMixedIssues flag is turned on, or when you're on level 1.


It can be aliased to no-return or never-return in docblocks. Note: it replaced the old empty type that used to exist in Psalm

This is the bottom type in PHP's type system. It's used to describe a type that has no possible value. It can happen in multiple cases:

  • the actual never type from PHP 8.1 (can be used in docblocks for older versions). This type can be used as a return type for functions that will never return, either because they always throw exceptions or always exit()
  • an union type that have been stripped for all its possible types. (For example, if a variable is string|int and we perform a is_bool() check in a condition, the type of the variable in the condition will be never as the condition will never be entered)
  • it can represent a placeholder for types yet to come — a good example is the type of the empty array [], which Psalm types as array<never, never>, the content of the array is void so it can accept any content
  • it can also happen in the same context as the line above for templates that have yet to be defined