C# String.Format() Equivalent in PHP?

You could use the sprintf function:

$filter = "content:%1$s title:%1$s^4.0 path.title:%1$s^4.0 description:%1$s ...";
$filter = sprintf($filter, "Cheese");

Or you write your own function to replace the {i} by the corresponding argument:

function format() {
    $args = func_get_args();
    if (count($args) == 0) {
        return;
    }
    if (count($args) == 1) {
        return $args[0];
    }
    $str = array_shift($args);
    $str = preg_replace_callback('/\\{(0|[1-9]\\d*)\\}/', create_function('$match', '$args=".var_export($args, true)."; return isset($args[$match[1]]) ? $args[$match[1]] : $match[0];'), $str);
    return $str;
}