PHP + Imagick = 脚本标题过早结束？

Question

我的图像目录中有以下 .htaccess 文件：

RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^(.*)\.jpg /images/image.php?%{REQUEST_FILENAME}

以及处理请求的脚本：

<?php
/*
 * Watermark module
 * Uses ImageMagick lib
 * 
 */

// The name of the watermark image. Should be in the same directory
// as the image.php
define('WATERMARK_IMAGE', 'watermark.png');

// Watermark images with larger width than this value (pixel)
// Set to 0 (Zero) to watermark all images
define('WATERMARK_THRESHOLD_WIDTH', 218);

$filename = $_SERVER['QUERY_STRING'];


// If the requested file doesn't exist, return HTTP 404
// Should not happen, as the htaccess handles that
if (file_exists($filename))
{
    // Get the last modified property of the source file for caching
    $file_last_modified = filemtime($filename);
    // Expirese in two months
    $expires = time() + (60 * 24 * 60 * 60);

    // Checking last modified date, if it's the same, then return 304 "Not Modified"
    // No body response is generated.
    if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) 
        && strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $file_last_modified)
    {
        header("HTTP/1.1 304 Not Modified");
    }
    else
    {
        $requested_image = new Imagick($filename);

        // If the marker image doesn't exist, then return the original image
        if (file_exists(WATERMARK_IMAGE))
        {

            $watermark = new Imagick(WATERMARK_IMAGE);

            // Get original image's dimensions
            $requested_image_width = $requested_image->getImageWidth();
            $requested_image_height = $requested_image->getImageHeight();

            // Get watermark image's dimensions
            $watermark_width = $watermark->getImageWidth();
            $watermark_height = $watermark->getImageHeight();

            // Calculate watermark position
            // Current position: center - center
            $position_x = ($requested_image_width - $watermark_width)/2;
            $position_y = ($requested_image_height - $watermark_height)/2;

            // Only watermark images larger than the threshold
            if ($requested_image_width > WATERMARK_THRESHOLD_WIDTH)
            {
                $requested_image->compositeImage($watermark, imagick::COMPOSITE_OVERLAY, $position_x, $position_y);
            }

            // Destroy the marker image
            $watermark->destroy();
        }

        // Set the headers
        header("Pragma: public");
        header("Content-type: image/jpeg");
        header("Expires: " . gmdate("D, d M Y H:i:s", $expires) . " GMT");
        header("Last-Modified: " . gmdate("D, d M Y H:i:s", $file_last_modified) . " GMT");

        // Return the response image
        echo $requested_image;

        // Destroy the temporary image
        $requested_image->destroy();
    }
}
else
{
    header('HTTP/1.1 404 Not Found');
}

/* End of file image.php */
/* Location: ./images/image.php */

我的站点正在 Apache Web 服务器上运行，最近它生成以下内容：

该站点锁定，我无法加载任何内容页 5-8 分钟。有时它会生成错误 500 或错误 503，有时则不会。错误日志在有问题的时间段中包含“脚本标头提前结束”行。

这种情况每 3-4 天发生一次，但一天中的时间有所不同，它发生在周日早上和工作日下午。

我真诚的问题是： 此代码中是否存在可能导致我的问题的严重问题/缺陷/漏洞？ （我没有任何理由认为问题是由脚本引起的，没有证据，但我很绝望，没有想法。）