Complex escape string

I have this line of code:

$q = "This is the string that will go into the query:
{${mysql_real_escape_string($_GET['searchstring'])}}";

What happens then is the user supplies 'foo' as the search string, and I get
a debug notice "Undefined variable: foo". Why is it treating the value as an
identifier and how do I make it do what I actually want it to do? This is on
PHP5, latest release.