private function _writeNoise()
{
$codeSet = '2345678abcdefhijkmnpqrstuvwxyz';
for ($i = 0; $i < 10; $i++) {
//杂点颜色
$noiseColor = imagecolorallocate($this->_image, mt_rand(150, 225), mt_rand(150, 225), mt_rand(150, 225));
for ($j = 0; $j < 5; $j++) {
// 绘杂点
imagestring($this->_image, 5, mt_rand(-10, $this->imageW), mt_rand(-10, $this->imageH), $codeSet[mt_rand(0, 29)], $noiseColor);
}
}
}