php做验证码输出图片显示的是乱码，怎么破？

<?phprequire_once 'string.func.php';function verifyImage($type = 1, $length = 4, $pixel = 0, $line = 0, $session_name = 'verify') {	session_start ();	// 创建画布	$width = 80;	$height = 28;	$image = imagecreatetruecolor ( $width, $height );	$white = imagecolorallocate ( $image, 255, 255, 255 );	$black = imagecolorallocate ( $image, 0, 0, 0 );	// 用填充矩形填充画布	imagefilledrectangle ( $image, 1, 1, $width - 2, $height - 2, $white );	$chars = buildRandomString ( $type, $length );	$_SESSION [$session_name] = $chars;	$fontfiles = array (			"ARIAL.TTF",			"CALIBRI.TTF" 	);	for($i = 0; $i < $length; $i ++) {		$size = mt_rand ( 14, 18 );		$angle = mt_rand ( - 15, 15 );		$x = 5 + $i * $size;		$y = mt_rand ( 20, 26 );		$fontfile = "../fonts/" . $fontfiles[mt_rand(0, count($fontfiles) - 1)];		$color = imagecolorallocate ( $image, mt_rand ( 50, 90 ), mt_rand ( 80, 200 ), mt_rand ( 90, 180 ) );		$text = substr ( $chars, $i, 1 );		imagettftext ( $image, $size, $angle, $x, $y, $color, $fontfile, $text );	}	// 判断是否加干扰点	if ($pixel) {		for($i = 0; $i < $pixel; $i ++) {			imagesetpixel ( $image, mt_rand ( 0, $width - 1 ), mt_rand ( 0, $height - 1 ), $black );		}	}	// 判断是否加干扰线	if ($line) {		for($i = 0; $i < $line; $i ++) {			$color = imagecolorallocate ( $image, mt_rand ( 50, 90 ), mt_rand ( 80, 200 ), mt_rand ( 90, 180 ) );			imageline ( $image, mt_rand ( 0, $width - 1 ), mt_rand ( 0, $height - 1 ), mt_rand ( 0, $width - 1 ), mt_rand ( 0, $height - 1 ), $color );		}	}	header ( "Content-Type:image/gif" );	imagegif ( $image );	imagedestroy ( $image );}