问Captcha镜像在webhost上“损坏”，但在localhost上没有

EN

<?php header('Content-type: image/jpeg'); ?>
<?php session_start(); ?>
<?php

$text = $_SESSION['captcha'];

//Image specs
$fontsize = 28;
$imagewidth = 130;
$imageheight = 50;

$image = imagecreate($imagewidth, $imageheight);
imagecolorallocate($image, 75, 146, 178);
$textcolor = imagecolorallocate($image, 255, 255, 255);

//Set fonts
$font1 = 'fonts/font1.ttf';
$font2 = 'fonts/font2.ttf';
$font3 = 'fonts/font3.ttf';

//Create 15 random lines
for ($x=1; $x<=15; $x++) {
    $linecolor = imagecolorallocate($image, 206, 228, 244);
    $x1 = rand(1,110);
    $y1 = rand(1,50);
    $x2 = rand(1,110);
    $y2 = rand(1,50);
    imageline($image, $x1, $y1, $x2, $y2, $linecolor);
}

//randomize font
$randfont = rand(1,3);
if ($randfont == 1) { 
    $font = $font1; 
} elseif($randfont == 2) { 
    $font = $font2; 
} else {
    $font = $font3;
}

//Randomize text angle
$angle = rand(-3,3);

imagettftext($image, $fontsize, $angle, 10, 32, $textcolor, $font, $text);
imagejpeg($image);

?>