1. The original paper from Bell Labs on UTF-8
2. A tutorial on character code issues – the MUST READ
3. Mojibake
4. Character Sets / Character Encoding Issues
5. Handling UTF-8 with PHP
6. Migrating to Unicode
7. http://en.wikipedia.org/wiki/UTF-8
8. Unicode block
9. Unicode Cheat Sheet
10. http://htmlpurifier.org/docs/enduser-utf8.html
11. What every programmer absolutely, positively needs to know about encodings and character sets to work with text
12. https://www.sitepoint.com/brin[...]-with-portable-utf8/
13. https://www.utf8-chartable.de/
14. W3C: Character Model for the World Wide Web: String Matching
15. https://unicode.org
    1. Unicode Regular Expressions
    2. Unicode Bidirectional Algorithm
    3. Unicode Security Considerations
    4. Unicode Normalization Forms
16. http://www.utf8everywhere.org/
17. Quotes

PHP 7.x/8.x

• Strings literals in PHP are still fundamentally composed of bytes.
• It is up to developers to deal with character encoding issues using mbstring, iconv, uconverter, etc.
• The intl extension wraps a lot of the functionality that was originally going to be a part of PHP 6 for use in PHP 7.x/8.x.
• PHP 7 helps by adding the inline UTF-8 literal syntax \u{[0-9A-Fa-f]+} and IntlChar class.