Added fixes to be compatible with PHP 8.1

[andrewbess]

This pull request (PR) provides fixes for the extension to make it compatible with PHP 8.1.
The fixes provides backward compatibility with previous versions PHP.

The fixes provided below

• the necessary constraints have been fixed if composer.json file
• composer dependencies have been updated to be compatible with PHP 7.3, 7.4, 8.0, 8.1
• return types have been fixed using attribute #[\ReturnTypeWillChange], please see this link
• The error "PHP Deprecated: explode(): Passing null to parameter W3 DOM Level 2 changes #2 ($string) of type string is deprecated" have been fixed. Please see this RFC document

Test results
Tests have been run on the local environment with PHP 7.3, 7.4, 8.0, 8.1RC6. Please see testdox results below

testdox73.txt
testdox74.txt
testdox80.txt
testdox81.txt

[g105b]

Hi @andrewbess,

This looks great. Thank you so much for your contribution. I will look over this now and once merged, I'll release it to 2.2.2.

Thanks,
Greg.