prevent users from input random string in php code example

Example 1: php random string

function rand_str() {
    $characters = '0123456789-=+{}[]:;@#~.?/>,<|\!"£$%^&*()abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $randomstr = '';
    for ($i = 0; $i < random_int(50, 100); $i++) {
      $randomstr .= $characters[rand(0, strlen($characters) - 1)];
    }
    return $randomstr;
  }

Example 2: random string generator php

<?php
    function RandomString()
    {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $randstring = '';
        for ($i = 0; $i < 10; $i++) {
            $randstring = $characters[rand(0, strlen($characters))];
        }
        return $randstring;
    }

    RandomString();
    echo $randstring;