$replace = array(" ",".",",","'","@");
$newString = str_replace($replace,"_",$join);

$join = "the original string i'm parsing through";

$newString = preg_replace('/[^a-z0-9]/i', '_', $join);

preg_replace('/[^a-zA-Z0-9]/', "_", $join);

preg_replace('/W/', '_', $join);