在图像调整大小脚本(GD）期间释放内存

我有以下代码，它从HTML <form>获取$_POST ed图像并上传它们。然后，它会重新调整所有图像的大小（最多两次），并将这些较小的图像保存到同一目录中。

但是，在处理较大的文件时，内存不足。

我的问题不是"我如何防止X错误发生？"，而是"我如何释放这个脚本正在使用的内存，以便它可以继续？"

<?php
if (!isset($_POST['SiteID']) || !filter_var($_POST['SiteID'], FILTER_VALIDATE_INT)) {
    header("Location: ./");
    exit();
}
$SiteID = $_POST['SiteID'];
if (!isset($_POST['GalleryID']) || !filter_var($_POST['GalleryID'], FILTER_VALIDATE_INT)) {
    header("Location: ./?SiteID=" . $SiteID . "&error=upload");
    exit();
}
$GalleryID = $_POST['GalleryID'];
$ImageFile = $_FILES['ImageFile'];
$Extensions = array("jpg", "jpeg");
$Timestamp = date("U");
require_once("db.php");
for ($X = 0; $X < count($_FILES['ImageFile']['name']); $X++) {
    $Extension = strtolower(pathinfo($ImageFile['name'][$X], PATHINFO_EXTENSION));
    if (in_array($Extension, $Extensions)) {
        if ($ImageFile['error'][$X] === UPLOAD_ERR_OK) {
            if ($ImageFile['size'][$X] <= 5 * 1024 * 1024) {
                $Filename = $GalleryID . "-" . $Timestamp . "-" . $X . "." . $Extension;
                move_uploaded_file($ImageFile['tmp_name'][$X], "uploads/" . $Filename);
                mysql_query("INSERT INTO NewGalleryImages SET GalleryID = $GalleryID, ImageFile = '$Filename', ImageCaption = ''");
                $Dimensions = getimagesize("uploads/" . $Filename);
                $Width = ($Dimensions[0] > 768 ? 768 : $Dimensions[0]);
                $Height = ($Dimensions[1] / $Dimensions[0]) * $Width;
                $Expl = explode(".", $Filename);
                $LargeFilename = $Expl[0] . "_gallery." . $Expl[1];
                $Source = "uploads/" . $Filename;
                $Canvas = imagecreatetruecolor($Width, $Height);
                $LargeImage = imagecreatefromjpeg($Source);
                imagecopyresampled($Canvas, $LargeImage, 0, 0, 0, 0, $Width, $Height, $Dimensions[0], $Dimensions[1]);
                imagejpeg($Canvas, "uploads/" . $LargeFilename, 100);
                imagedestroy($Canvas);
                $Width = 200;
                $Height = ($Dimensions[1] / $Dimensions[0]) * $Width;
                $Expl = explode(".", $Filename);
                $ThumbFilename = $Expl[0] . "_thumb." . $Expl[1];
                $Source = "uploads/" . $Filename;
                $Canvas = imagecreatetruecolor($Width, $Height);
                $ThumbImage = imagecreatefromjpeg($Source);
                imagecopyresampled($Canvas, $ThumbImage, 0, 0, 0, 0, $Width, $Height, $Dimensions[0], $Dimensions[1]);
                imagejpeg($Canvas, "uploads/" . $ThumbFilename, 100);
                imagedestroy($Canvas);
            } else {
                mysql_close();
                header("Location: ./?SiteID=" . $SiteID . "&error=size");
                exit();
            }
        } else {
            mysql_close();
            header("Location: ./?SiteID=" . $SiteID . "&error=unknown");
            exit();
        }
    } else {
        mysql_close();
        header("Location: ./?SiteID=" . $SiteID . "&error=type");
        exit();
    }
}
mysql_close();
header("Location: ./?SiteID=" . $SiteID . "&uploaded");
exit();
// var_dump($_POST);
// var_dump($_FILES);
// var_dump($ImageFile);
?>

我在PHP文档中读到了imagedestroy()，并尝试在几个地方将其添加到我的脚本中，但1）我不确定我是否破坏了正确的资源，2）我不知道它是否真的在清除内存。

我目前在服务器上有以下设置：

post_max_size = 64M
upload_max_filesize = 10M
max_input_time = 60
max_input_nesting_level = 64
max_execution_time = 120
memory_limit = 512M
max_input_vars = 1000

这是我收到的错误信息：

Fatal error: Allowed memory size of 536870912 bytes exhausted
(tried to allocate 29440 bytes) in
/var/www/vhosts/example.com/httpsdocs/gallery-upload.php
on line 36

我觉得奇怪的是，无论我把memory_limit增加多少，它需要分配的"额外"总是一样的：29440 bytes。

如果我单独上传图像，脚本运行正常，但一次上传两个以上会导致错误。超过512MB的内存来调整两个图像的大小似乎太多了，不是吗？还是内存仍然不足？我不确定这台服务器能腾出多少如何减少总内存使用量