core-bundle/src/EventListener/MergeHttpHeadersListener.php
private function fetchHttpHeaders(): void
    {
        $this->headers = array_merge($this->headers, $this->headerStorage->all());
        $this->headerStorage->clear();
    }
private function fetchHttpHeaders(): void
    {
        $this->headers = array_merge($this->headers, $this->headerStorage->all());
        $this->headerStorage->clear();
    }
Fetches and stores HTTP headers from PHP.