PHP验证码效能的实现

/**
	 *产生验证码图片
	 */
	public function actionVerfiycode() {
		Header ( "Content-type: image/gif" );
		$border = 0; //是否要边框 1要:0不要
		$how = 4; //验证码位数
		$w = $how * 15; //图片宽度
		$h = 20; //图片高度
		$fontsize = 5; //字体大小
		$alpha = "abcdefghijkmnopqrstuvwxyz"; //验证码内容1:字母
		$number = "0123456789"; //验证码内容2:数字
		$randcode = ""; //验证码字符串初始化
		srand ( ( double ) microtime () * 1000000 ); //初始化随机数种子
		$im = ImageCreate ( $w, $h ); //创建验证图片
		$bgcolor = ImageColorAllocate ( $im, 255, 255, 255 ); //设置背景颜色
		ImageFill ( $im, 0, 0, $bgcolor ); //填充背景色
		if ($border) {
			$black = ImageColorAllocate ( $im, 0, 0, 0 ); //设置边框颜色
			ImageRectangle ( $im, 0, 0, $w - 1, $h - 1, $black ); //绘制边框
		}
		for($i = 0; $i < $how; $i ++) {
			$alpha_or_number = mt_rand ( 0, 1 ); //字母还是数字
			$str = $alpha_or_number ? $alpha : $number;
			$which = mt_rand ( 0, strlen ( $str ) - 1 ); //取哪个字符
			$code = substr ( $str, $which, 1 ); //取字符
			$j = ! $i ? 4 : $j + 15; //绘字符位置
			$color3 = ImageColorAllocate ( $im, mt_rand ( 0, 100 ), mt_rand ( 0, 100 ), mt_rand ( 0, 100 ) ); //字符随即颜色
			ImageChar ( $im, $fontsize, $j, 3, $code, $color3 ); //绘字符
			$randcode .= $code; //逐位加入验证码字符串
		}
		
		for($i = 0; $i < 5; $i ++) //绘背景干扰线
		{
			$color1 = ImageColorAllocate ( $im, mt_rand ( 0, 255 ), mt_rand ( 0, 255 ), mt_rand ( 0, 255 ) ); //干扰线颜色
			ImageArc ( $im, mt_rand ( - 5, $w ), mt_rand ( - 5, $h ), mt_rand ( 20, 300 ), mt_rand ( 20, 200 ), 55, 44, $color1 ); //干扰线
		}
		for($i = 0; $i < $how * 40; $i ++) //绘背景干扰点
		{
			$color2 = ImageColorAllocate ( $im, mt_rand ( 0, 255 ), mt_rand ( 0, 255 ), mt_rand ( 0, 255 ) ); //干扰点颜色 
			ImageSetPixel ( $im, mt_rand ( 0, $w ), mt_rand ( 0, $h ), $color2 ); //干扰点
		}
		$session=new CHttpSession;
		$session->open();	
		Yii::app()->session->add('randcode',$randcode);
		/*绘图结束*/
		Imagegif ( $im );
		ImageDestroy ( $im );
		/*绘图结束*/
	
	}