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*/		}