Warning: implode() [function.implode]: Invalid arguments passed

It happens when $ret hasn't been defined. The solution is simple. Right above $tags = get_tags();, add the following line:

$ret = array();

You can try

echo implode(', ', (array)$ret);

You are getting the error because $ret is not an array.

To get rid of the error, at the start of your function, define it with this line: $ret = array();

It appears that the get_tags() call is returning nothing, so the foreach is not run, which means that $ret isn't defined.