PHP code running twice under certain conditions

Question

I'm having this strange issue where when certain conditions are met my PHP code is run twice. My code adds a watermark to a JPEG image (done in PHP to allow me to change the watermark quickly if necessary), and resized if requested. To speed up requests the processed images are cached. Whenever the image is loaded I'm incrementing a view counter, however when the following happens the counter is increased by 2:

• width or height is set in a GET request
• the output MIME type is set (commenting out the header() call runs the script as expected
• imagecreatefromjpeg() is called and the image is resized - when these lines are commented out the script runs normally. The script also runs normally if the image is not resized.
• the browser is Firefox. Internet Explorer 11 behaves normally, confirmed by my access logs. (I haven't tested other browsers.)

Here's my code:

<?php
require_once("../config.php");
if (empty($_GET['photoid'])) {
    header("Location: /photos");
    die();
}
$photoId = intval($_GET['photoid']);
$photo = getPhotoData($photoId);
if ($photo == null) {
    http_response_code(404);
    echo "That photo doesn't exist.";
} else {
    $filename = $photo['photo_filename'];

    $largePath = "large/";
    $cachePath = $largePath . "view_cache/";
    $cache = false;

    if (!isset($_GET['admin']) && !isset($_GET['thumb']) && !isset($_GET['tile'])) {
        incrementPhotoViewCount($photoId);
    }

    if (isset($_GET['height'])) {
        list($originalWidth, $originalHeight) = getimagesize($largePath . $photo['photo_filename']);
        $height = min(intval($_GET['height']), $originalHeight);
        if (file_exists($cachePath . $height . "h" . $filename)) {
            $im = imagecreatefromjpeg($cachePath . $height . "h" . $filename);
            $cache = true;
        } else {
            $im = resizeImage($largePath . $photo['photo_filename'], $originalWidth, $height);
        }
    } else if (isset($_GET['width'])) {
        list($originalWidth, $originalHeight) = getimagesize($largePath . $photo['photo_filename']);
        $width = min(intval($_GET['width']), $originalWidth);
        if (file_exists($cachePath . $width . "w" . $filename)) {
            $im = imagecreatefromjpeg($cachePath . $width . "w" . $filename);
            $cache = true;
        } else {
            $im = resizeImage($largePath . $photo['photo_filename'], $width, $originalHeight);
        }
    } else {
        if (file_exists($cachePath . $filename)) {
            $im = imagecreatefromjpeg($cachePath . $filename);
            $cache = true;
        } else {
            $im = imagecreatefromjpeg($largePath . $photo['photo_filename']);
        }
    }

    if (!$cache && ((!isset($height) || $height > 400) && (!isset($width) || $width > 350))) {
        $watermark = imagecreatefrompng("../images/watermark.png");

        //Preserve original watermark transparency
        imagealphablending($watermark, true); // setting alpha blending on
        imagesavealpha($watermark, true); // save alphablending setting (important)

        //Set the margins for the watermark and get the height/width of the stamp image
        $marginLeft = 20;
        $marginBottom = 20;
        $sx = imagesx($watermark);
        $sy = imagesy($watermark);

        //Merge the stamp onto our photo
        imagecopy($im, $watermark, $marginLeft, imagesy($im) - $sy - $marginBottom, 0, 0, $sx, $sy);
    }

    //Output the image and free memory
    header("Content-Type: image/jpeg");
    imagejpeg($im);
    if (!$cache) {
        if (isset($_GET['height'])) {
            imagejpeg($im, $cachePath . $height . "h" . $filename);
        } else if (isset($_GET['width'])) {
            imagejpeg($im, $cachePath . $width . "w" . $filename);
        } else {
            imagejpeg($im, $cachePath . $filename);
        }
    }
    imagedestroy($im);
}

function resizeImage($file, $w, $h, $crop = false) {
    list($width, $height) = getimagesize($file);
    $r = $width / $height;
    if ($crop) {
        if ($width > $height) {
            $width = ceil($width-($width*abs($r-$w/$h)));
        } else {
            $height = ceil($height-($height*abs($r-$w/$h)));
        }
        $newwidth = $w;
        $newheight = $h;
    } else {
        if ($w/$h > $r) {
            $newwidth = $h*$r;
            $newheight = $h;
        } else {
            $newheight = $w/$r;
            $newwidth = $w;
        }
    }
    $src = imagecreatefromjpeg($file);
    $dst = imagecreatetruecolor($newwidth, $newheight);
    imagecopyresampled($dst, $src, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);

    return $dst;
}

Here's the code that updates the view counter:

function incrementPhotoViewCount($photoId) {
    $photoId = intval($photoId);

    if (!($stmt = $GLOBALS['mysqli']->prepare("UPDATE `Photo` SET `view_count` = `view_count` + 1 WHERE `photo_id` = ?"))) {
        error_log("Prepare failed: (" . $GLOBALS['mysqli']->errno . ") " . $GLOBALS['mysqli']->error);
        return false;
    }

    if (!$stmt->bind_param("i", $photoId)) {
        error_log("Binding parameters failed: (" . $stmt->errno . ") " . $stmt->error);
        return false;
    }

    if (!$stmt->execute()) {
        echo "Execute failed: (" . $stmt->errno . ") " . $stmt->error;
        return false;
    }
}

What's going on here?

Answer 1

Some extra late-night Googling answered my question - turns out Firefox has a bug which causes it to load dynamically generated images twice when loaded directly. It's fine when loaded through an <img> tag, and all other browsers behave normally. Something to look at for anyone else having this issue :)