Ir varianti:
<?php
$str = '/fol1/fol2/fol3/';
#1 variants
echo implode('/', array_splice(explode('/', $str), 0, -2)).'/';
#2 variants
if (preg_match('@^(.*/)[^/]+/$@', $str, $m)) {
echo $m[1];
}
#3 variants
echo substr($str, 0, strrpos(substr($str, 0, -1), '/')+1);
?>