Yup - to keep with good coding practices ;) Strings should be wrapped in either ' ... ' or " ..." and numbers can be left as is :)

Should be '...' for stings and "..." for html or holding a variable togeather if you want to goto good codeing practices. ;)

' ... ' 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.

$mystring = '<a href="$link">test url</a>';

Should work?

in that case it will just print exactly that as php does not parse the string for variables :)

Unless of course that string gets passed through eval(); later on ;)