php做的验证码输出出现乱码，怎么解

<?php 
function vcode($width=120,$height=40,$fontSize=30,$countElement=5,$countPixel=100,$countLine=4){
header('Content-type:image/jpeg');
$element=array('a','b','c','d','e','f','g','h','i','j','k','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
$string='';
for ($i=0;$i<$countElement;$i++){
$string.=$element[rand(0,count($element)-1)];
}
$img=imagecreatetruecolor($width, $height);
$colorBg=imagecolorallocate($img,rand(200,255),rand(200,255),rand(200,255));
$colorBorder=imagecolorallocate($img,rand(200,255),rand(200,255),rand(200,255));
$colorString=imagecolorallocate($img,rand(10,100),rand(10,100),rand(10,100));
imagefill($img,0,0,$colorBg);
for($i=0;$i<$countPixel;$i++){
imagesetpixel($img,rand(0,$width-1),rand(0,$height-1),imagecolorallocate($img,rand(100,200),rand(100,200),rand(100,200)));
}
for($i=0;$i<$countLine;$i++){
imageline($img,rand(0,$width/2),rand(0,$height),rand($width/2,$width),rand(0,$height),imagecolorallocate($img,rand(100,200),rand(100,200),rand(100,200)));
}
//imagestring($img,5,0,0,'abcd',$colorString);
imagettftext($img,$fontSize,rand(-5,5),rand(5,15),rand(30,35),$colorString,'font/ManyGifts.ttf',$string);
imagejpeg($img);
imagedestroy($img);
return $string;
}
?>