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