<?php
$filename = "ip.list";
$prefix = "2a04:5200:8";

function generate_ipv6($prefix){
  function generate_ipv6_block() {
    $seed = str_split('1234567890abcdef');
    shuffle($seed);
    $block = join("", $seed); // Symbol array to string
    $block = substr($block, 0, 4);
    return $block;
  }

  $a = generate_ipv6_block();
  $b = generate_ipv6_block();
  $c = generate_ipv6_block();
  $d = generate_ipv6_block();
  $e = generate_ipv6_block();

  return "{$prefix}:{$a}:{$b}:{$c}:{$d}:{$e}";
}

function generate_unique_ipv6($prefix, $iplist) {
  $ipv6 = null; // Scope

  do {
    $ipv6 = generate_ipv6($prefix);
  } while(in_array($ipv6, $iplist));

  return $ipv6;
}

if(file_exists($filename)){
  $ips = trim(file_get_contents($filename));
  $ips = explode("\n", $ips);
}
else{
  $ips = [];
}

$ip = generate_unique_ipv6($prefix, $ips);
$ips[] = $ip;
file_put_contents($filename, implode("\n", $ips));

echo $ip;