两次结果输出不一样，老师解释下，为啥第一次打印的是字符串

$stack ='';

$collections = [1,2,3,4,5,6,7];

foreach($collections as $key => $collection) {

$stack[$key] = $collection . '_' . $key;

}

var_dump($stack);//打印出来是一个字符串，理解？string(7) "1234567"

echo '<br>';

$stack =[];

$collections = [1,2,3,4,5,6,7];

foreach($collections as $key => $collection) {

$stack[$key] = $collection . '_' . $key;

}

var_dump($stack);//输出是数组

/*

array(7) {

  [0]=>

  string(3) "1_0"

  [1]=>

  string(3) "2_1"

  [2]=>

  string(3) "3_2"

  [3]=>

  string(3) "4_3"

  [4]=>

  string(3) "5_4"

  [5]=>

  string(3) "6_5"

  [6]=>

  string(3) "7_6"

*/