What does check_markup() actually do? The following code:

var_dump(check_markup('This is a test', 'plain_text'));


outputs:

string(22) "&lt;p&gt;This is a test&lt;/p&gt;"


Why does it do this?
check_markup() will:


  Run all the enabled filters on a piece of text.


Since one of those filters will probably be the HTML corrector, you'll get the &lt;p&gt;&lt;/p&gt; tags wrapping your text string.