<?php 

$sql = 'select fielda, fieldb, fieldc from table1
order by field, fieldb, fieldc
union all
select field1, field2, field3 from table2
order by field1, field2 desc, field3 asc';


print_r($sql2 = invert_order($sql));


function invert_order($str){
    $re = '/(?<=order\sby\s)(.*)(?=)/mi';

    preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
    
    $mat = $matches[sizeof($matches)-1]; //-- get only last order field list
    $mat=$mat[sizeof($mat)-1];
    $mat=explode(",",$mat);
    
    
    for($i=0; $i<sizeof($mat); $i++){   //-- reverse each pair of order field/direction 
        $duet = preg_split("/\s+/", trim($mat[$i]));
        if (sizeof($duet)<2) $duet[1]="";
        switch(strtolower($duet[1])) {
            case "desc":
                $duet[1] = "asc";
                break;
            default:
                $duet[1] = "desc";
                break;
        }
        $mat[$i] = implode(" ",$duet);
    }
    
    $re2 = '/(order\s+by\s+.*)*$/i';    //-- replace last order by with new inverted order by:
    $subst = "order by ".implode(", ",$mat);
    $result = preg_replace($re2, "", $str);
    return $result . " $subst";
}


?>