substr_replace：整个字符串(从这里结束) 替换成这个变量 从什么开始(默认从下标0开始)
<!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);
$new_substring="cyg&liwen";
echo $new_string=substr_replace($_GET['username'],$new_substring,6);
?>