replace "asc" with "desc" on first occurrence only of ORDER BY SQL clause

[ppowell]

$orderBy = 's.app_date desc, s.last_name asc, s.first_name asc, s.mi asc';
   if ($_REQUEST['willDesc']) {
    $ascArray = array('asc' => 'desc', 'desc' => 'asc'); 	// ARRAY OF ALL ORDERING POSSIBILITIES
    $junk = preg_match('/([\s\t]+)a|[de]sc(,?.*$)/i', $orderBy, $matchArray);
    $orderBy = substr($orderBy, 0, strpos($orderBy, $matchArray[1])) . ' ' . $ascArray[$matchArray[1]] . 
               substr($orderBy, strpos($orderBy, $matchArray[1]) + strlen($matchArray[1]), strlen($orderBy));
   } 

Basic premise:

I have a SQL "ORDER BY" clause that will be configured like $orderBy 's value. However, the very first occurrence of "desc" might instead be "asc". If the very first occurrence is "asc", it must become "desc"; likewise, if the very first occurrence is "desc", it must become "asc".

I tried Regular Expressions but the pattern failed every single time to match and replace, so I gave up and tried a string function/RegExp code combination, also to no avail.

I am not sure how to make this work so I need help figuring out how to do this.

Thanx
Phil

[Wynder]

This's is a pain... it only works one way:

$orderBy = 's.app_date desc, s.last_name asc, s.first_name asc, s.mi asc';

if($orderBy = preg_replace("^desc^", "asc", $orderBy, 1))
{
   $pos = strpos($orderBy, 'asc') + 4;
}


if($newPos = strpos($orderBy, "asc", $pos))
{
   $orderBy = substr($orderBy, 0, $newPos) . "desc, " . substr($orderBy, $newPos + 4);
   echo $orderBy;
}

This returns: s.app_date asc, s.last_name desc, s.first_name asc, s.mi asc

But, if you return the asc and desc in the original string, it breaks -- not a complete solution, but it might give you some more options to try.

[ppowell]

I managed to get it to work, but using a function I saw online (and literally guessed as to how to fix it, I do not see why this works!):

if (!function_exists('change_first_order_flag')) {
 function change_first_order_flag($orderBy){
  $tmp = explode('__SPLIT__HERE__', preg_replace('`(asc|desc)`i', '__SPLIT__HERE__$1', $orderBy)); 
  if (count($tmp) > 1) { 
   // there are at least 2 elements 
   // therefore, it was in there at least once 
   if (substr(strtolower($tmp[1]), 0, 3) == 'des') {  // it was in descending order 
    $tmp[1] = 'as' . substr($tmp[1], 3); 
   } else {  // it was in ascending order 
    $tmp[1] = 'desc' . substr($tmp[1], 3); 
   } 
  }  

  return join($tmp);
 }
}

Perhaps what is confusing me is the pattern matching using backticks.

Phil