AES encrypt PHP 5.6 to PHP 7.3

Question

I would like to transform the code below code PHP 5.6 to PHP 7.3, We are more grateful to that person who provides us some suggestions on learning encryption in PHP on a more detailed level so we can try to convert that code itself.

function generateRandomString($length = 10)
{
    
    $characters       = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString     = '';
    for($i = 0; $i < $length; $i++)
    {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    
    return $randomString;
}

function strToHex($string)
{
    
    $hex = '';
    for($i = 0; $i < strlen($string); $i++)
    {
        $ord     = ord($string[$i]);
        $hexCode = dechex($ord);
        $hex     .= substr('0' . $hexCode, -2);
    }
    
    return strToUpper($hex);
}

function AESencrypt($decrypted, $password = 'password', $salt = 'salt_key')
{
    
    $block     = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $padding   = $block - (strlen($decrypted) % $block);
    $decrypted .= str_repeat(chr($padding), $padding);
    $encrypted = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $password, $decrypted, MCRYPT_MODE_CBC, $password));
    $encrypted = hash('SHA256', $encrypted . '|' . $salt, true);
    
    return strToHex($encrypted);
}

$agentid   = "EU01EU22INB000";
$mercode   = "BPP";
$username  = "BPP_01";
$userpass  = "BPP_01";
$storecode = "BPP_01";
$ipaddr    = "103.235.104.168";
$billerid    = '00000NATTQ';
$bbpstel     = 'QH01WD3737';
$bbpstel1    = $bbpstel;
$stringParts = str_split($bbpstel1);
sort($stringParts);
$sortval = implode('', $stringParts);
$sort = $sortval;
$Billfetch = "BillPay" . generateRandomString();

$hashkey   = AESencrypt($Billfetch . "|" . $mercode . "|" . $agentid . "|" . $billerid . "|" . $sort, "password", "salt_key");
echo $Billfetch."----".$hashkey."<br/>";
exit;