src/IpAddress.php
protected function normalizeProxyHeader( $header )
    {
        $header = strtoupper( $header );
        $header = str_replace( '-', '_', $header );
        if ( 0 !== strpos( $header, 'HTTP_' ) )
        {
            $header = 'HTTP_' . $header;
        }
        return $header;
    }
protected function normalizeProxyHeader( $header )
    {
        $header = strtoupper( $header );
        $header = str_replace( '-', '_', $header );
        if ( 0 !== strpos( $header, 'HTTP_' ) )
        {
            $header = 'HTTP_' . $header;
        }
        return $header;
    }
Normalize a header string

Normalizes a header string to a format that is compatible with
$_SERVER

@param  string $header
@return string