wp-includes/class-wp-hook.php:554Returns the key of the current element.
mixed #[ReturnTypeWillChange] public function key() { return key( $this->callbacks ); }Introduced in 4.7.0. One change between 6.7.7 and 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
mixed to int|null.verified against sourcesrc/wp-includes/class-wp-hook.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.