Примеры
$c = []; foreach ( $t as $p ) { $c[] = $p->id; } $c = join( ', ', $c );
$str = ''; foreach ( $t as $p ) { $str.= $p->id; } //print_r($str); 12345
$arr = array(); foreach ( $t as $p ) { $arr[] = $p->id; } //print_r($array); array(0=>1, 1=>2..)