understudySpecify the call by making it.

No method names in strings. One engine, two runner adapters and two static analysers.

The call is the specification

when(fn () => $repo->find(1))->returns($book) — the method is called, so a rename, a wrong argument type or a dropped parameter is a type error before the test runs.

Matchers that keep their types

Arg::any(), Arg::same(), Arg::satisfies() and Arg::captor() read through the contract, so a matcher in the wrong position is caught by Psalm and PHPStan.

Failure messages that point

The asterisks mark the argument that differed, and object aliases say which of two look-alike instances the call carried.

Order when you need it

ordered(), verifySequence() and expectSequence() — the last one fails at the call that broke the protocol, not in teardown.

Real objects underneath

delegate() runs the real object for everything not stubbed, and records it. wire() builds the subject with its doubles in place.

Bring your own runner

A Testo interceptor, a PHPUnit trait, Pest, or no framework at all — verification is the same engine underneath.

Five packages, one engine

understudy — the engine: doubles, stubbing, expectations, verification, transcripts. Depends on nothing but PHP.
understudy-testo — verification and reset wired into Testo's lifecycle, with Fiber isolation.
understudy-phpunit — the same, through a trait; works under Pest.
understudy-psalm — a Psalm plugin that types the specification closure and reports misuse.
understudy-phpstan — the same checks as PHPStan rules, under the understudy.* identifiers.

Install the engine plus exactly the adapter your suite already uses. The static analysers are independent of both: either one can be added on its own.

The message tells you which one it was

An object argument is matched by identity, so two instances never match however equally they read. The message has to be able to say which of the two reasons a call failed — and it does:

text

Understudy `BookRepository` expected `save(App\Book#1 {title: 'Dune'})` to be called
exactly 2 times, but it was called 1 time.

The following calls to `save` were made during this test:
    save(App\Book#1 {title: 'Dune'})
    save(*App\Book#2 {title: 'Dune'}*)

#1 and #2 are aliases numbered within one message, in order of first appearance — not object ids, which are reused after a collection and would print differently on the next run. The asterisks mark what differed.

See the Cookbook for this and four other real incidents, each with a script you can run.