src/Requests/RequestHeaderParser.php
public function parseHeaders(array $server): array
    {
        $headers = [];

        foreach ($server as $key => $value) {
            $uppercasedKey = \strtoupper($key);

            if (isset(self::$specialCaseHeaders[$uppercasedKey]) || \strpos($uppercasedKey, 'HTTP_') === 0) {
                $value = (array)$value;
                $headers[self::normalizeName($key)] = $value;
            }
        }

        return $headers;
    }
public function parseHeaders(array $server): array
    {
        $headers = [];

        foreach ($server as $key => $value) {
            $uppercasedKey = \strtoupper($key);

            if (isset(self::$specialCaseHeaders[$uppercasedKey]) || \strpos($uppercasedKey, 'HTTP_') === 0) {
                $value = (array)$value;
                $headers[self::normalizeName($key)] = $value;
            }
        }

        return $headers;
    }
Parses headers from the $_SERVER super global

@param array $server The $_SERVER super global
@return array The mapping of header names => values