First-class callable syntax replaces four spellings

A callable could be written four ways, all of them strings or arrays that no tool could follow through a rename.

// before, all of these
'strlen'
[$this, 'handle']
[Order::class, 'fromArray']
Closure::fromCallable([$this, 'handle'])

// 8.1
strlen(...)
$this->handle(...)
Order::fromArray(...)

$lengths = array_map(strlen(...), $names);

The literal ... is not a placeholder — it is the syntax, and it produces a Closure bound at the point of creation, which means visibility is checked there rather than at call time. That is the substantive difference from the array form: a private method can be passed out as a first-class callable from inside the class and the resulting closure works, where [$this, 'privateMethod'] would have failed wherever it was eventually invoked.