Union, intersection, DNF types

Parenthesized (A&B)|C

Union means or while intersection means and. DNF form groups an intersection in brackets, then joins it with a union.

Read brackets first: the value must match the full bracket group or match the trailing option.

interface HasId { public function id(): int; } interface HasName { public function name(): string; } function show((HasId & HasName)|string $v): string { if (is_string($v)) { return $v; } return $v->name(); } echo show("hi");

never type

Type never marks a function that does not return at all. It always throws or exits instead of giving back a value.

Use never for guard helpers that stop bad paths so the main code stays flat.

function abort(string $msg): never { throw new RuntimeException($msg); } function root(int $x): float { if ($x < 0) { abort("negative"); } return sqrt($x); } echo root(9);

mixed vs explicit

Type mixed accepts anything and skips checks. Explicit unions list only the shapes the code really handles.

Start explicit for public APIs and keep mixed for truly open slots like debug printers.

function dumpAny(mixed $v): string { return get_debug_type($v); } function addNum(int|float $v): float { return $v + 1; } echo dumpAny("hi"); echo addNum(2);

Standalone null false true

Modern PHP lets null, false and true stand alone as types. They describe exact results like a failed search or a strict flag.

A function returning false instead of bool promises that true never appears in that slot.

function findId(array $ids, int $want): int|false { foreach ($ids as $id) { if ($id === $want) { return $id; } } return false; } var_dump(findId([1, 2], 2));

Next: Readonly deep dive

Article author: Arthur Isaev