wp-includes/rest-api/class-wp-rest-request.php:213$keystringstring|null public function get_header( $key ) { $key = $this->canonicalize_header_name( $key ); if ( ! isset( $this->headers[ $key ] ) ) { return null; } return implode( ',', $this->headers[ $key ] ); }Introduced in 4.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rest-api/class-wp-rest-request.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.