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. W3C: Character Model for the World Wide Web: String Matching
14. https://unicode.org
    1. Unicode Regular Expressions
    2. Unicode Bidirectional Algorithm
    3. Unicode Security Considerations
    4. Unicode Normalization Forms
15. http://www.utf8everywhere.org/
16. 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 7/8.
• PHP 7 helps by adding the inline UTF-8 literal syntax \u{[0-9A-Fa-f]+} and IntlChar class.

Unicode & IDNA

RFC 3986 neither supports IDNA, nor non-ASCII characters. WHATWG URL supports IDNA and Unicode characters, and it explicitly suggests that browsers should render the host component by displaying Unicode characters.
The recommendation is not just for user-friendliness: it's necessary for security reasons, alleviating the human risk factor in exploits. E.g. “xn--google.com” could deceive the uninitiated reader that it is a Google domain, however the IDNA domain decodes to “䕮䕵䕶䕱.com” in fact.