Make seems_utf8() RFC 3629 compliant.

[gitlost]

seems_utf8() should be made ​RFC 3629 compliant. Currently it accepts overlong sequences and surrogates, which will cause PHP functions expecting valid UTF-8 strings to fail.

[desrosj]

Related: #29717.

Trac Ticket: Core-38044

## Overview

• This pull request introduces the seems_utf8 function, which validates UTF-8 encoded strings according to the specifications outlined in RFC 3629. This implementation ensures that only valid UTF-8 sequences are accepted, effectively safeguarding applications against invalid input.

## Key Features:

• UTF-8 Encoding Compliance:

• The function adheres strictly to the UTF-8 encoding rules defined in RFC 3629, which allows for a maximum of 4 bytes per character.

• Handling of Single and Multi-byte Sequences:

• It correctly identifies and processes single-byte (0x00 to 0x7F) and multi-byte sequences (2 to 4 bytes), ensuring that each byte in a multi-byte sequence begins with the appropriate prefix.

• Validation of Leading Bytes:

• The function checks leading bytes to determine the number of continuation bytes required:

• 0xC0 for 2-byte sequences
• 0xE0 for 3-byte sequences
• 0xF0 for 4-byte sequences

• It explicitly rejects any leading bytes starting with 0xF8 or 0xFC, as these indicate sequences that exceed the valid UTF-8 range.

• Control Over Overlong Sequences:

• The function rejects overlong sequences, ensuring that the encoding does not use more bytes than necessary to represent a character, thereby preventing potential security issues.

• Surrogate Pair Handling:

• It prevents the inclusion of invalid surrogate pairs (U+D800 to U+DFFF) in the encoded string, in compliance with the restrictions specified in RFC 3629.

• Zero Byte Validation:

• The function checks for invalid overlong sequences specifically for U+0000, adhering to best practices for UTF-8 validation.

• Comprehensive Error Handling:

• Each check returns false for invalid cases, ensuring that any non-compliant string is effectively filtered out, thereby providing robustness against various encoding issues.

## Conclusion

• The seems_utf8 function is a comprehensive implementation that ensures full compliance with RFC 3629 standards. By validating UTF-8 strings effectively, it enhances the integrity and security of applications that rely on proper character encoding. This pull request aims to integrate this functionality, providing developers with a reliable tool for UTF-8 validation.

Hi @desrosj , can you please take a look into this PR. Thanks.