I would like to be able to generate a fill in the blank from a word missing 2 letters randomly

$word = "superman";

I would like to be able to randomly choose 2 letters from what ever word is in $word and build a fill in the blank box for me to be able to answer

example some how random picks the p and the a su_erm_n comes up so I would fill in the first box with p and the second one with a I guess with a form

$wordfinished = "su" . $fillinblank1 . "erm" . $fillinblank2 . "n";

if ($wordfinshed == $word) {
echo "congrats";
}
else{
echo "try again";
}

I am just learning php I have some things that I have done that are very complicated but was having a hard time with the random stuff any help would help me learn this