PHP strip punctuation

Let's say I have this:

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

and I want to strip punctuation so it outputs as:

hello_is_stackoverflow_a_helpful_website_yes

How can I do that?

# to keep letters & numbers
$s = preg_replace('/[^a-z0-9]+/i', '_', $s); # or...
$s = preg_replace('/[^a-z\d]+/i', '_', $s);

# to keep letters only
$s = preg_replace('/[^a-z]+/i', '_', $s); 

# to keep letters, numbers & underscore
$s = preg_replace('/[^\w]+/', '_', $s);

# same as third example; suggested by @tchrist; ^\w = \W
$s = preg_replace('/\W+/', '_', $s);

for string

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

result (for all examples) is

Hello_is_StackOverflow_a_helpful_website_Yes_

Enjoy!

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).

Without regular expressions:

<?php
  $hello = "Hello, is StackOverflow a helpful website!? Yes!"; // original string
  $unwantedChars = array(',', '!', '?'); // create array with unwanted chars
  $hello = str_replace($unwantedChars, '', $hello); // remove them
  $hello = strtolower($hello); // convert to lowercase
  $hello = str_replace(' ', '_', $hello); // replace spaces with underline
  echo $hello; // outputs: hello_is_stackoverflow_a_helpful_website_yes
?>

I'd go with something like this:

$str = preg_replace('/[^\w\s]/', '', $str);

I don't know if that's more broad than you're looking for, but it sounds like what you're trying to do.

I also notice you've replaced spaces with underscores in your sample. The code I'd use for that is:

$str = preg_replace('/\s+/', '_', $str);

Note that this will also collapse multiple spaces into one underscore.