If T1 to T21 is the only thing you have in your array then do the following:
$trades = implode(',', $_POST);
Else you could extract those values by walking the array and taking the values if they start with T. For example:
$ExtractedResults = array();
function GetTFields(&$item1, $key){
global $ExtractedResults;
if(substr($key, 0, 1) == 'T' && is_numeric(substr($key, 1)) && (int)(substr($key, 1)) >= 1 && (int)(substr($key, 1)) <= 21){
$ExtractedResults[$key] = $item1;
}
}
array_walk($_POST, 'GetTFields');
$trades = implode(',', $ExtractedResults);
Cheers