验证码类 支持数字，字母，汉字，混合

width	 =  $width;
			$this->height 	 =  $height;
			$this->codeNum 	 =  $codeNum;
			$this->codeType	 =  $codeType;
			$this->fontSize  =  $fontSize;
			$this->fontType	 =  $fontType;
			$this->codeStr	 =  $codeStr;
			$this->strNum = strlen($this->codeStr)/3-1;
			$this->imageType =  $imageType;
			$this->checkCode =  $this->getCheckCode();
		}

      //+--------------------------------------------------------------------------------
      //* 生成验证码字符
      //+--------------------------------------------------------------------------------
      //* @return string
      //+--------------------------------------------------------------------------------
		public function __toString(){			
			$string = implode('', $this->getCheckCode());
			$_SESSION["code"]=$string;  //加到session中
			$this->getImage();              //输出验证码
			return '';
		}	
		protected function getCheckCode(){
			$string = array();				
			switch($this->codeType){
				case 1:
					//数字字符串
					$string =  array_rand(range(0,9), $this->codeNum);
					break;
				case 2:
					//大字母字符串
					$string = array_rand(array_flip(range('A', 'Z')), $this->codeNum);
					break;
				case 3:
					//汉字字符串
					
					for($i=0; $icodeNum); $i++){
						$start = mt_rand(0, $this->strNum);
						$string[$i]= self::msubstr($this->codeStr,$start);
					}
					break;
				case 4:
					//混合字符串
					for($i=0; $icodeNum); $i++){
						$rand=mt_rand(0,2);
						switch($rand){
							case 0:
								$ascii = mt_rand(48,57);
								$string[$i] = sprintf('%c',$ascii);
								break;