system/HTTP/Header.php
public function getValueLine(): string
	{
		if (is_string($this->value))
		{
			return $this->value;
		}
		else if (! is_array($this->value))
		{
			return '';
		}

		$options = [];

		foreach ($this->value as $key => $value)
		{
			if (is_string($key) && ! is_array($value))
			{
				$options[] = $key . '=' . $value;
			}
			else if (is_array($value))
			{
				$key       = key($value);
				$options[] = $key . '=' . $value[$key];
			}
			else if (is_numeric($key))
			{
				$options[] = $value;
			}
		}

		return implode(', ', $options);
	}
public function getValueLine(): string
	{
		if (is_string($this->value))
		{
			return $this->value;
		}
		else if (! is_array($this->value))
		{
			return '';
		}

		$options = [];

		foreach ($this->value as $key => $value)
		{
			if (is_string($key) && ! is_array($value))
			{
				$options[] = $key . '=' . $value;
			}
			else if (is_array($value))
			{
				$key       = key($value);
				$options[] = $key . '=' . $value[$key];
			}
			else if (is_numeric($key))
			{
				$options[] = $value;
			}
		}

		return implode(', ', $options);
	}
Retrieves a comma-separated string of the values for a single header.

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.

@see https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2