It’s all in the documentation!
$str="Str 1: %1$s - Str 2: %2$s - Str 2 again: %2$s";
echo sprintf($str, "I'm string 1", "My name is string 2");
Note: Use single quotes for the format string otherwise you’ll get PHP Notice: Undefined variable: s in /path/to/tofile:line
You can also escape the $ with a \ if you are using double quotes.