substr:整个字符串 从哪里开始(第一个是下标0) 最后是哪里(比如写8那8-1=7就对了)
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title></title>
</head>
<body>
<form method="get">
<input type="text" name="username" id="">
<input type="submit" value="提交">
</form>
</body>
</html>
<?php
error_reporting(0);
echo $username=substr($_GET['username'],0,8);
?>