[Gavin B.]

' ... ' is a simple string which is not parsed by PHP for variables, etc.
" ... " is a complex string which is parsed (and is therefore slightly less efficient).

$mystring = '<a href="'.$link.'">Test URL</a>';
$mystring = "<a href=\"$link\">Test URL</a>";

both produce the same result, however the first example is more efficiant and (IMO) easier to read and work with.