Как сохранить результат foreach как переменную ?

foreach ( $t as $p ) {
  $c.= $p->id .', ';
}

$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..)

$str = '';
foreach ( $t as $p ) {
		$str = $p->id;
}
//print_r($str); last 5