关于中文验证码的问题，不解

<?php session_start();$_SESSION['code']= 0;header("Content-type: image/png"); $im = @imagecreatetruecolor(70, 27) or die("建立图像失败"); //创建图片$background_color = imagecolorallocate($im, 250, 250, 250); //图片添加背景imagefill($im,0,0,$background_color);$border_color = imagecolorallocate($im,0,0,0); //边框色imagerectangle($im,0,0,69,26,$border_color);//噪音线for($i = 0;$i < 7;$i++){	$x1 = rand(3,25);	$y1 = rand(2,26);//开始位置		$x2 = rand(45,68);//结束位置	$y2 = rand(2,26);	$line_color = imagecolorallocate($im,rand(190,255),rand(190,255),rand(190,255)); //噪音线颜色	imageline($im,$x1,$y1,$x2,$y2,$line_color); }//显示的文字$font_size = 18;$first_num = 0;//图片中显示的数字，包含中文$second_num = 0;$first_value = 0;//后台预算的数字$second_value = 0;$hanzi_num = array("一","二","三","四","五","六","七","八","九","十");//$str_HanZiOrNot = rand(0,1);//是否是汉字的判断$result = 0;$arrCapcher = array("+","-");$capter = $arrCapcher[rand(0,1)];switch ($capter){	case "+":		//产生第一个数字		if(rand(0,1) == 0){// ==0表示纯数字			$first_num = rand(2,10);			$first_value = $first_num;		}		else{       //汉字			$index_ = rand(0,10);			$first_num = $hanzi_num[$index_];			$first_value = $index_ + 1;		}		//产生第二个数字		if(rand(0,1) == 0){//      ==0纯数字			$second_num = rand(0,10);			$second_value = $second_value;		}		else{			$index_ = rand(0,10);			$second_num = $hanzi_num[$index_];			$second_value = $index_ + 1;		}				$result = $first_value + $second_value;		break;	case "-":		//产生第一个数字		if(rand(0,1) == 0){// ==0表示纯数字			$first_num = rand(2,10);			$first_value = $first_num;		}		else{       //汉字			$index_ = rand(0,10);			$first_num = $hanzi_num[$index_];			$first_value = $index_ + 1;		}		//产生第二个数字		if(rand(0,1) == 0){//      ==0纯数字			$second_num = rand(0,$first_value);			$second_value = $second_value;		}		else{			$index_ = rand(0,$first_value-1);			$second_num = $hanzi_num[$index_];			$second_value = $index_ + 1;		}				$result = $first_value - $second_value;		break;}$x = rand(2,5);$arr_X_Y = array(array($x,rand(1,3)),array($x+$font_size,rand(1,4)),array($x+2*$font_size,rand(1,3)),array($x+3*$font_size,rand(1,4)));$str_char = array($first_num,$capter,$second_num,"=");for($i = 0;$i< 4;$i++){	$text_color = imagecolorallocate($im,rand(50,120),rand(50,120),rand(50,120)); 	imagechar($im,$font_size,$arr_X_Y[$i][0],$arr_X_Y[$i][1],$str_char[$i],$text_color); }$_SESSION['code'] = $result;imagepng($im); imagedestroy($im);?>