public function add(string $header): self
    {
        foreach ($this->getAll() as $tmp) {
            if ($tmp === $header) {
                throw new Exception("The '{$header}' header has already been added.");
            }
        }
        $this->headerList[] = $header;
        return self::$instance;
    }
public function add(string $header): self
    {
        foreach ($this->getAll() as $tmp) {
            if ($tmp === $header) {
                throw new Exception("The '{$header}' header has already been added.");
            }
        }
        $this->headerList[] = $header;
        return self::$instance;
    }
public function add ( string $ header ) : self { foreach ( $ this -> getAll ( ) as $ tmp ) { if ( $ tmp === $ header ) { throw new Exception ( "The '{$header}' header has already been added." ) ; } } $ this -> headerList [ ] = $ header ; return self :: $ instance ; }
It adds the header to the stack.
@param string $header The page header.
@return self.
@throws \nadir2\core\Exception It's thrown if the passed header has already
been added earlier.
It adds the header to the stack .