Captcha不显示，在不同的浏览器中获得不同的错误

Question

I run this on one server and I can see the captcha image.

On an other server I am not getting the image.

Chrome reports no response received, while mozilla showing connection timed out.

When I comment out the line header('content-type'..., both are displaying garbage values, so I doubt, that the script does not generate proper header content type on that particular server.

<?php
ob_start();
session_start();        
$image_width = 120;
$image_height = 35;
$characters_on_image = 6;
$dir        =   dirname(__FILE__);

function hexrgb ($hexstr)
{
  $int = hexdec($hexstr);

  return array("red" => 0xFF & ($int >> 0x10),
               "green" => 0xFF & ($int >> 0x8),
               "blue" => 0xFF & $int);
}

$font = $dir.'/monofont.ttf';
$possible_letters = '23456789bcdfghjkmnpqrstvwxyz';
$random_dots = 0;
$random_lines = 20;
$captcha_text_color="0x142864";
$captcha_noice_color = "0x142864";
$code = '';
$i = 0;
while ($i < $characters_on_image) { 
    $code .= substr($possible_letters, mt_rand(0, strlen($possible_letters)-1), 1);
    $i++;
}
$_SESSION['captcha']=$code;     
$font_size  = $image_height * 0.75;
$image      = @imagecreate($image_width, $image_height);


/* setting the background, text and noise colours here */
$background_color   = imagecolorallocate($image, 255, 255, 255);

$arr_text_color = hexrgb($captcha_text_color);
$text_color     = imagecolorallocate($image, $arr_text_color['red'],$arr_text_color['green'], $arr_text_color['blue']);

$arr_noice_color = hexrgb($captcha_noice_color);
$image_noise_color  = imagecolorallocate($image, $arr_noice_color['red'],$arr_noice_color['green'], $arr_noice_color['blue']);


/* generating the dots randomly in background */
for( $i=0; $i<$random_dots; $i++ ) {
    imagefilledellipse($image, mt_rand(0,$image_width),mt_rand(0,$image_height), 2, 3, $image_noise_color);
}


/* generating lines randomly in background of image */
for( $i=0; $i<$random_lines; $i++ ) {
    imageline($image, mt_rand(0,$image_width), mt_rand(0,$image_height),mt_rand(0,$image_width), mt_rand(0,$image_height), $image_noise_color);
}


/* create a text box and add 6 letters code in it */
$textbox    = imagettfbbox($font_size, 0, $font, $code); 
$x          = ($image_width - $textbox[4])/2;
$y          = ($image_height - $textbox[5])/2;
imagettftext($image, $font_size, 0, $x, $y, $text_color, $font , $code);


header('Content-Type: image/png');
imagepng($image);
imagedestroy($image);
exit;
?>