PHP/HTML问题将div内容放在一起

<?php
$div1 = "<div>内容1</div>";
$div2 = "<div>内容2</div>";
$div3 = "<div>内容3</div>";

$combinedDiv = $div1 . $div2 . $div3;

echo $combinedDiv;
?>

<div id="parentDiv">
  <div>内容1</div>
  <div>内容2</div>
  <div>内容3</div>
</div>

<style>
  .container {
    display: flex;
    flex-wrap: wrap;
  }
  
  .container div {
    width: 100px;
    height: 100px;
    border: 1px solid black;
    margin: 5px;
  }
</style>

<div class="container">
  <div>内容1</div>
  <div>内容2</div>
  <div>内容3</div>
</div>