如何使用php填充manifest.json文件？

I have a manifest.json file that loads correctly when placed in the root directory of my wesite. Instead of it being a static file, I would like to use php variables from within my site to populate the values inside of the manifest.

I can't find any information on this in the spec, and I'm not sure if it's possible.

I've tried switching the name of my manifest to manifest.php and used header('Content-Type: application/json') within it.

Inside my index file head:

<script src="<?php echo $SITE_URL;?>/main.js"></script>
<script src="<?php echo $SITE_URL;?>/sw.js"></script>
<link rel="manifest" href="<?php echo $SITE_URL;?>/manifest.php">

Inside my manifest.php:

<?php header('Content-Type: application/json');
echo "
{
  \"name\": \"$SiteName\",
  \"gcm_user_visible_only\": true,
  \"short_name\": \"$Name\",
  \"description\": \"$PageDescription.\",
  \"start_url\": \"/index.php\",
  \"display\": \"standalone\",
  \"orientation\": \"portrait\",
  \"background_color\": \"$darkblue\",
  \"theme_color\": \"#f0f0f0\",
  \"icons\": [{
    \"src\": \"logo-load.png\",
    \"sizes\": \"96x96 128x128 144x144\",
    \"type\": \"image/png\"
  },{
    \"src\": \"logo-icon.png\",
    \"sizes\": \"48x48 72x72\",
    \"type\": \"image/png\"     
  }]
}
";
?>

The variables $SiteName, $Name, $PageDescription, $darkblue, etc are all defined within my document head before the manifest.php is loaded.

Is what I'm trying possible?

Main question I have is where are these constants pulled from? Sometimes people set them at the server host level and import them (environment variables). Another option (sort of blurs with what you are doing) is to parse an INI file.

I would do something like the following:

<?php

$siteName = 'foo';
$name = 'bar';
$pageDescription = 'baz';

$manifest = [
    "name" => $siteName,
    "gcm_user_visible_only" => true,
    "short_name" => $name,
    "description" => $pageDescription,
    "start_url" => "/index.php",
    "display" => "standalone",
    "orientation" => "portrait",
    "background_color" => $darkblue,
    "theme_color" => "#f0f0f0",
    "icons" => [
        "src" => "logo-load.png",
        "sizes"=> "96x96 128x128 144x144",
        "type" => "image/png"
    ],
    "src" => "logo-icon.png",
    "sizes" => "48x48 72x72",
    "type" => "image/png"
];

header('Content-Type: application/json');
echo json_encode($manifest);