When adding a new status a number of strings need to be defined. The names are magical. The formula should be explained. Suggestion:

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The variable should be named $s_<STATUS>__bug_button where <STATUS> is the same string following the colon after the constant number in the $s_status_enum_string defined above. Spaces should be replaced by underscores.

Personally, I think it'd be a good idea to separate <STATUS> from the rest of the name with double underscores. That'd make it more clear what to change (albeit uglier).

Even nicer would probably be to use a two dimensional array (or better yet a dictionary keyed on status).

Something along the lines of
$s_statusstring[TEST]['buttontext'] = 'The bug is ready for test';