src/Message.php
protected function setHeader($name, $value)
	{
		if (!is_array($value)) {
			$value = [$value];
		}

		$this->headerNames[strtolower($name)] = $name;
		array_merge($this->headers[$name] = $value);

		return $this;
	}
protected function setHeader($name, $value)
	{
		if (!is_array($value)) {
			$value = [$value];
		}

		$this->headerNames[strtolower($name)] = $name;
		array_merge($this->headers[$name] = $value);

		return $this;
	}
Set the header.

@param string $name
@param string|string[] $value
@return $this