ubuntu 的PHP服务器,验证码图片不显示字符,怎么回事

<?phpfunction getAuthImage($text) {	$im_x = 160;	$im_y = 40;	$im = imagecreatetruecolor($im_x,$im_y);	$text_c = ImageColorAllocate($im, mt_rand(0,100),mt_rand(0,100),mt_rand(0,100));	$tmpC0=mt_rand(100,255);	$tmpC1=mt_rand(100,255);	$tmpC2=mt_rand(100,255);	$buttum_c = ImageColorAllocate($im,$tmpC0,$tmpC1,$tmpC2);	imagefill($im, 16, 13, $buttum_c);	$font = 't1.ttf';	for ($i=0;$i<strlen($text);$i++)	{		$tmp =substr($text,$i,1);		$array = array(-1,1);		$p = array_rand($array);		$an = $array[$p]*mt_rand(1,10);//角度		$size = 28;		imagettftext($im, $size, $an, 15+$i*$size, 35, $text_c, $font, $tmp);	}	$distortion_im = imagecreatetruecolor ($im_x, $im_y);	imagefill($distortion_im, 16, 13, $buttum_c);	for ( $i=0; $i<$im_x; $i++) {		for ( $j=0; $j<$im_y; $j++) {			$rgb = imagecolorat($im, $i , $j);			if( (int)($i+20+sin($j/$im_y*2*M_PI)*10) <= imagesx($distortion_im)&& (int)($i+20+sin($j/$im_y*2*M_PI)*10) >=0 ) {				imagesetpixel ($distortion_im, (int)($i+10+sin($j/$im_y*2*M_PI-M_PI*0.1)*4) , $j , $rgb);			}		}	}	//加入干扰象素;	$count = 160;//干扰像素的数量	for($i=0; $i<$count; $i++){		$randcolor = ImageColorallocate($distortion_im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));		imagesetpixel($distortion_im, mt_rand()%$im_x , mt_rand()%$im_y , $randcolor);	}	$rand = mt_rand(5,30);	$rand1 = mt_rand(15,25);	$rand2 = mt_rand(5,10);	for ($yy=$rand; $yy<=+$rand+2; $yy++){		for ($px=-80;$px<=80;$px=$px+0.1)		{			$x=$px/$rand1;			if ($x!=0)			{				$y=sin($x);			}			$py=$y*$rand2;			imagesetpixel($distortion_im, $px+80, $py+$yy, $text_c);		}	}	//设置文件头;	Header("Content-type: image/JPEG");	//以PNG格式将图像输出到浏览器或文件;	ImagePNG($distortion_im);	//销毁一图像,释放与image关联的内存;	ImageDestroy($distortion_im);	ImageDestroy($im);}function make_rand($length="32"){//验证码文字生成函数	$str="ABCDEFGHIJKLMNOPQRSTUVWXYZ";	$result="";	for($i=0;$i<$length;$i++){		$num[$i]=rand(0,25);		$result.=$str[$num[$i]];	}	return $result;}//输出调用$checkcode = make_rand(4);session_start();//将随机数存入session中$_SESSION['helloweba_gg']=strtolower($checkcode);getAuthImage($checkcode);?>

<?phpfunction getAuthImage($text) {	$im_x = 160;	$im_y = 40;	$im = imagecreatetruecolor($im_x,$im_y);	$text_c = ImageColorAllocate($im, mt_rand(0,100),mt_rand(0,100),mt_rand(0,100));	$tmpC0=mt_rand(100,255);	$tmpC1=mt_rand(100,255);	$tmpC2=mt_rand(100,255);	$buttum_c = ImageColorAllocate($im,$tmpC0,$tmpC1,$tmpC2);	imagefill($im, 16, 13, $buttum_c);	$font = 't1.ttf';	for ($i=0;$i<strlen($text);$i++)	{		$tmp =substr($text,$i,1);		$array = array(-1,1);		$p = array_rand($array);		$an = $array[$p]*mt_rand(1,10);//角度		$size = 28;		imagettftext($im, $size, $an, 15+$i*$size, 35, $text_c, $font, $tmp);	}	$distortion_im = imagecreatetruecolor ($im_x, $im_y);	imagefill($distortion_im, 16, 13, $buttum_c);	for ( $i=0; $i<$im_x; $i++) {		for ( $j=0; $j<$im_y; $j++) {			$rgb = imagecolorat($im, $i , $j);			if( (int)($i+20+sin($j/$im_y*2*M_PI)*10) <= imagesx($distortion_im)&& (int)($i+20+sin($j/$im_y*2*M_PI)*10) >=0 ) {				imagesetpixel ($distortion_im, (int)($i+10+sin($j/$im_y*2*M_PI-M_PI*0.1)*4) , $j , $rgb);			}		}	}	//加入干扰象素;	$count = 160;//干扰像素的数量	for($i=0; $i<$count; $i++){		$randcolor = ImageColorallocate($distortion_im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));		imagesetpixel($distortion_im, mt_rand()%$im_x , mt_rand()%$im_y , $randcolor);	}	$rand = mt_rand(5,30);	$rand1 = mt_rand(15,25);	$rand2 = mt_rand(5,10);	for ($yy=$rand; $yy<=+$rand+2; $yy++){		for ($px=-80;$px<=80;$px=$px+0.1)		{			$x=$px/$rand1;			if ($x!=0)			{				$y=sin($x);			}			$py=$y*$rand2;			imagesetpixel($distortion_im, $px+80, $py+$yy, $text_c);		}	}	//设置文件头;	Header("Content-type: image/JPEG");	//以PNG格式将图像输出到浏览器或文件;	ImagePNG($distortion_im);	//销毁一图像,释放与image关联的内存;	ImageDestroy($distortion_im);	ImageDestroy($im);}function make_rand($length="32"){//验证码文字生成函数	$str="ABCDEFGHIJKLMNOPQRSTUVWXYZ";	$result="";	for($i=0;$i<$length;$i++){		$num[$i]=rand(0,25);		$result.=$str[$num[$i]];	}	return $result;}//输出调用$checkcode = make_rand(4);session_start();//将随机数存入session中$_SESSION['helloweba_gg']=strtolower($checkcode);getAuthImage($checkcode);?>