How to intercept the content in the form using PHP string?

<code>        <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
            <input type="text" name="name" class="a1-input" placeholder="名字">
            <div class="a1-btn-min"><button class="a1-btn waves-effect waves-light">YES</button></div>
        </form></code>

<code><?php

$name = "";
$name_str = mb_strlen($name,"UTF8");

if ($_SERVER["REQUEST_METHOD"] == "POST") {
      if (empty($_POST["name"])) {
            echo "<script>alert('你并没有写你的名字...')</script>";
      } else if($name_str >= 10){
            echo "<script>alert('这貌似不是一个名字吧~！')</script>";
      } else{
            $name = test_input($_POST["name"]);
      }
}
  
function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}

?></code>