Check User Data for Better WHMCS Security

This hook checks user data, in order to prevent the user from registering or later editing his data using:

fake or disposable email addresses (using https://www.mailcheck.ai/) free API service
special characters
too long strings

<?php
/**
* check user data
*
* This hook checks user data, in order to prevent the user from registering
* or later editing his data using:
* - fake or disposable email addresses (using https://www.mailcheck.ai/)
* free API service
* - special characters
* - too long strings
* @copyright DomainRegister
* https://domainregister.international
* info@domainregister.it
* License: GNU General Public License v3.0
*
*/

if (!defined("WHMCS"))
die("This file cannot be accessed directly");

function checkuserdata($vars) {
$email = $vars['email'];
$elements = ['firstname','lastname','companyname','address1','address2','city','state','postcode','country','phonenumber','tax_id'];

$usererrors = [];

foreach($elements as $element){
if (checkstring($vars[$element])){
array_push($usererrors, "Your ".$element." contains invalid characters");
}
if (strlen($vars[$element])>150){
array_push($usererrors, "Your ".$element." is too long");
}
}

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://api.mailcheck.ai/email/' . $email);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
curl_close($ch);
$response2=json_decode($response);
$debug = var_export($response2, true);
if ($response2->disposable OR !$response2->mx)
{ array_push($usererrors, "Your email address is not valid");
}

return $usererrors;
}

function checkstring ($string1)
{
$invalidchars=str_split("£$%&/()=!#@?^[]<>");
foreach($invalidchars as $character){
if (strchr($string1, $character))
{ return true;
}
}
return;
}