Generating random ref_id in PHP? - php

Question

i want to create different ref_id for each users. i tried it with using str_shuffle function. it generates but when i refresh the page it changes. also, it generates the same ref_id for every user. Here are my codes:**
$users = $db->get_results("SELECT * FROM users");
if($users ){
foreach($users as $u){
$str = '1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPUQRSTUVWXYZ';
$str_r = substr(str_shuffle($str), 0, 30);
$ref_id=$db->query("UPDATE users SET ref_id= '$str_r'") ;
echo "ref_id: $u->ref_id; ";
echo '<br';
}
}
i think i must use while but i didnt get any result. How can i fix this?**
Edit:
i solved the part of "generating different id for each users" but still changes every refreshing. How can i solve that part?
here are the new version of codes above:
$users = $db->get_results("SELECT * FROM users");
if($users){
echo '<div class="container">
<div class="card">
<div class="card-body">';
foreach($users as $u){
$str = '1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPUQRSTUVWXYZ';
$str_r = substr(str_shuffle($str), 0, 20);
$ref_id=$db->query("UPDATE users SET ref_id= '$str_r' where id='$u->id'") ;
echo "$u->id . users ref_id: $u->ref_id; ";
echo '<br>';
}
}
echo '</div></div></div>';