Expose a function to check whether a string is valid UTF-8

[stof]

Description

Checking whether a string is valid UTF-8 is a common need, as many projects require UTF-8 encoding (to be able to use the u modifier in regex for instance, among other use cases).

As zend_string already has internal optimizations to remember that a string is valid UTF-8, implementing such function in core would provide several benefits over a userland implementation (it can both read the flag for a fast-path answer and set the flag at the end of the slow path to remember the validity for next usages of the string).

This could be implemented as a function is_valid_utf8(string $string): bool function

[iluuu1994]

Isn't that pretty much what mb_check_encoding($string, 'UTF-8') already does, apart from not being in core required? It checks the encoding and adds the IS_STR_VALID_UTF8 flag if appropriate, and avoids the check again in the future. Apart from some internal strings, this flag is only added by ext-mbstring when checking encodings on-demand. There's another feature request to add the flag for compile time strings (GH-10853), but that would still require a runtime encoding check for dynamic strings.

Also related to https://wiki.php.net/rfc/unicode_text_processing.

[stof]

mb_check_encoding($string, 'UTF-8') is indeed doing that, but this introduces a dependency on ext-mbstring which might not be OK for many packages (depending on what they do with the UTF-8 string, they might not need mbstring for that).

In practice, many packages end up doing false !== preg_match('//u', $string) as a way to detect that, relying on the fact that PCRE fails for invalid UTF-8 when using the u modifier (and the fact that an empty regex should hopefully not fail for another weird reason, as it is a valid pattern and will not require backtracking that could reach the JIT limit).

[stof]

A quick github search for preg_match('//u' returns 7k results: https://github.com/search?q=%22preg_match%28%27%2F%2Fu%27%22&type=code

And this includes only cases using a single-quoted string and the / delimiter for the regex.

[mvorisek]

mb_check_encoding($string, 'UTF-8') is indeed doing that

I can confirm, both are working great, the mb_check_encoding is very fast since PHP 8.2 - https://github.com/atk4/ui/blob/6.0.0/src/HtmlTemplate/Value.php#L28-L33