src/System/Input.php
public static function getHeader($header, $defaultValue = '')
    {
        $header = strtolower(str_replace(['_', ' '], '-', $header));
        $headers = self::getHeaders();
        if (is_array($headers)) {
            foreach ($headers as $name => $value) {
                if (strtolower($name) == $header) {
                    return $value;
                }
            }
        }
        return $defaultValue;
    }
public static function getHeader($header, $defaultValue = '')
    {
        $header = strtolower(str_replace(['_', ' '], '-', $header));
        $headers = self::getHeaders();
        if (is_array($headers)) {
            foreach ($headers as $name => $value) {
                if (strtolower($name) == $header) {
                    return $value;
                }
            }
        }
        return $defaultValue;
    }
Get header.

@param string $header
@param string $defaultValue
@return mixed|string