[PHP] usort(): The argument should be an array

[Remember14a]

Can anyone comment and fix this error
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Warning: usort(): The argument should be an array in 
/home2/wwwabcde/public_html/search/searchfuncs.php on line 300
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below is part of the code which gives this error, the line highlighted in red 
is the point where error is pointing, 
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$row[4];

   $res[$i]['size'] = $row[5];

   $res[$i]['weight'] = $result_array[$row[0]];

   $i++;

  }
  usort($res, "cmp"); 

  echo mysql_error();

  $res['maxweight'] = $maxweight;

  $res['results'] = $results;

  return $res;

 /**/

 }

?>