<?php
$arr = array();
$arr['oassade'] = 'one&1';
$arr['asdas'] = 'tw&o2';
$arr['asdsads'] = '&three123';
$columns = array($arr);

foreach ($arr as $key => $value){
     $arr[$key]  = str_replace("&", "&amp;", $value);
}
print_r($arr);

Array
(
    [oassade] => one&1
    [asdas] => tw&o2
    [asdsads] => &three123
)