its AJAX – no document gets requested, just some sort of php script..
The option is to set a document/resource [blank template] to Content Type .php [alias: name.php]
and just call a [!snippet!] with the php code.
http://modxcms.com/forums/index.php/topic,37438.msg225891.html#msg225891
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http://www.devtrench.com/modx-chunk-cache-plugin/ )
