In a program ...

This works:
   $url = str_replace('$id',$id,$url." ");

But this doesnt: (It doesnt replace '$id' at all.)
  $url = str_replace('$id',$id,$url);

(It should work.)

$url is some string coming from a MySQL database, with '$id' in it (as text). This time at the end. If I assign the url manually it replaces fine, just not when it comes from a SQL datbase AND it's at the end of string. (So it's pretty hard to reproduce in just text.)