The reason the + is dissappearing is because + in a URL get is translated into a space. You can't stop this.
IF you are generating the URL that has "something+something" as a value, you can try something like this:
$mylink = "<a href=\"myfile.php?variable=" . urlencode("something+something") . "\">Click Here</a>";
This should come back as with a + sign as you originally wanted...
More info here: http://www.php.net/manual/en/function.urlencode.php
Also, take a look at $QUERY_STRING. It will be the unparsed query string, and will also have the + signs there, but you will need to parse it yourself.
Hope this helps,
-Josh B