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

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

		return isset($this->headers[$orig_name]);
	}
Determines whether a header exists.

@param string    $name

@return boolean