如何在scssphp中设置php变量

I try to set a php variable inside scssphp. But I don't know where my mistake is.

<?php header('content-type: text/css');
require_once 'scssphp-0.7.6/scss.inc.php';
use Leafo\ScssPhp\Compiler;
$scss = new Compiler();

$command = '';

if(get_field("background_color", "option")) {

$command = '$body-color: ' . the_field("background_color", "option") . ';';
$command .= 'body {
background-color: $body-color;
}';
}

echo $scss->compile($command);


?>