the following is the back-end code 
 I think $username= is a randomly generated string in the output, so write $username=$number; like this, but this is an execution error, how to do it? For advice, thank you 
<?php
$username=$_POST["username"];
$numbers = range (1,20); 
//
srand ((float)microtime()*1000000); 
shuffle ($numbers); 
//list
while (list(, $number) = each ($numbers)) { 
//echo "$number "; 
 $username=$number;
 $data="{username:"" . $username . ""}";//json
 echo json_encode($data);//json
?>