system/HTTP/Message.php
public function getHeaderLine(string $name): string
	{
		$orig_name = $this->getHeaderName($name);

		if (! array_key_exists($orig_name, $this->headers))
		{
			return '';
		}

		return $this->headers[$orig_name]->getValueLine();
	}
public function getHeaderLine(string $name): string
	{
		$orig_name = $this->getHeaderName($name);

		if (! array_key_exists($orig_name, $this->headers))
		{
			return '';
		}

		return $this->headers[$orig_name]->getValueLine();
	}
Retrieves a comma-separated string of the values for a single header.

This method returns all of the header values of the given
case-insensitive header name as a string concatenated together using
a comma.

NOTE: Not all header values may be appropriately represented using
comma concatenation. For such headers, use getHeader() instead
and supply your own delimiter when concatenating.

@param string $name

@return string