Semi-Newbie question about arrays

I'm trying to create a multidimensional array of values to update a Filemaker db. Edits to existing fields end in numbers (the record ID).I'm having a problem, probably obvious to anyone here in setting up the multidimensional array:

$updateEntries = array();     //stores existing db entries

$ord = -1; $recNum = 0; foreach ($_POST as $key => $value) { if ($value != "") {

               $lastPartPos  = strrchr($key, '_');
               $lastPart = substr($lastPartPos, 1);
               if (preg_match("/[0-9]+/" , $lastPart)) {  //if it's an existing update
                   $lastpartnum = strrpos($key, '_');
                   $firstnum = substr($key, 0, $lastpartnum);
                   $first = str_replace('_', '.', $firstnum);  //restore periods to field name and strip off record ID at end
                   if ($lastPart != $recNum) {
                       $recNum = $lastPart;
                       $ord++;
                       $updateEntries[$ord] = $recNum;
                   }
                   $updateEntries[$ord][] = array($first, $value); //populate nested arrays of each entry

etc...... throws: "PHP Fatal error: [] operator not supported for strings" How do I make $updateEntries[$ord] evaluate as an array? Thanks in advance RG