React\Gifsocket\GifEncoder::addFrame PHP Method

    public function addFrame($frame, $delay = 0)
    {
        $data = '';
        if (null === $this->initialFrame) {
            $this->initialFrame = $frame;
            $data .= $this->writeHeader($frame);
        }
        $sourceHeader = substr($frame, 0, 6);
        if (!in_array($sourceHeader, array('GIF87a', 'GIF89a'))) {
            throw new \InvalidArgumentException('Input frame must be a GIF');
        }
        $i = 13 + 3 * (2 << (ord($frame[10]) & 0x7));
        while (true) {
            $sectionType = $frame[$i];
            if ('!' === $sectionType && 'NETSCAPE' === substr($frame, $i + 3, 8)) {
                throw new \InvalidArgumentException('Input frame must not be animated');
            }
            if (';' === $sectionType) {
                break;
            }
            $i++;
        }
        $data .= $this->encodeFrame($frame, $delay);
        return $data;
    }