Really simple to fix the issue, however keep in mind that you should fork and commit your changes for each library you are using in their repositories to help others as well.
Let's say you have something like this in your code:
$str = "test";
echo($str{0});
since PHP 7.4 curly braces method to get individual characters inside a string has been deprecated, so change the above syntax into this:
$str = "test";
echo($str[0]);
