$string = "something here.";
$output = 'something here';

$output = rtrim($string, '.');

$string = "something here..";
echo preg_replace("/.$/","",$string);

<?php
$output = $string;
$last_char_pos = strlen($output) - 1;
if ($output[ $last_char_pos ] == '.') {
    $output[ $last_char_pos ] = null;
}
?>