Using variable in @SELECT query?

I am trying to use a page variable in a @SELECT statement and it is not working:

@SELECT pagetitle, id FROM modx_site_content WHERE parent = 0 AND id != "$content[’parent’]"

If I replace the variable with an ID it works.

Also on the documentation for the @SELECT binder it states that you can use the [+prefix+] placeholder to identify the table prefix. However this does not seem to work.
http://modxcms.com/select-binding.html

No, this is currently not possible; you are better off using @EVAL to issue the query and format proper results. And try [+PREFIX+].

[+PREFIX+] works. In the docs the image shows [+prefix+] and the text reads [+PREFIX+]...I believed the image, should have believed the text

I will use @EVAL instead. What is the best way to get the parent ID for the document being edited? Can I use the documentObject? Or the content array $content[’parent’]?