Expand Variables in String

Hello, Is it possible to force a string to expand variable names after the string has been set and before the variable's been defined without having to do a string replace or preg_replace? for example, <?php $str = "some string and some \$var plus other stuff"; echo $str."<br />"; $var = "Variable Contents"; echo $str."<br />"; $str_expanded = $str;//would like this to expand $var into $str_expanded echo $str_expanded."<br />"; ?> Thanks, dK `