PHP strip punctuation

Question

Let\'s say I have this:

$hello = \"Hello, is StackOverflow a helpful website!? Yes!\";

and I want to strip punctuation so it outputs as:

Answer 1

function strip_punctuation($string) {
    $string = strtolower($string);
    $string = preg_replace("/[:punct:]+/", "", $string);
    $string = str_replace(" +", "_", $string);
    return $string;
}

First the string is converted to lower case, then punctuation is removed, then spaces are replaced with underscores (this will handle one or more spaces, so if someone puts two spaces it will be replaced by only one underscore).