$word = "shshsh.shshsh"; print(rtrim($word, "sh."));
Output is an empty string when I would have expected it to output the entire string (since "sh." does not appear at the end in this case).
"sh." is a set of characters rather than a string to remove. It'll remove characters from the end of the
$word recursively as long as it's a character in
[s, h, .], until it hits one that isn't in this list, essentially removing everything in your case.