更改json文件中的变量值

Question

我有visit.json

{
"vid.1":"1",
"vid.2":"1",
"vid.3":"1",
"vid.4":"1",
"vid.5":"1"
}

和一个php页面

<?php
 $urlmovie = "visit.json";
 $movie = file_get_contents($urlmovie); 
 $visit = 'vid.4';
 $array=json_decode($movie,1);
 $vis=$array [$visit];
 $visitplus = $vis+1;
// some coding here to save visit.json "vid.4":".$vis+1." every time is access  
 echo $visitplus;
?>

如何用$ visitplus替换vid.4的值！