Add FILTER_VALIDATE_STRLEN for UTF-8 string length validation

[masakielastic]

This PR introduces a new filter validator FILTER_VALIDATE_STRLEN.

The validator checks the length of a UTF-8 string in Unicode code points and allows applications to enforce minimum and/or maximum string length.

API

New filter constant:

FILTER_VALIDATE_STRLEN

Supported options:

min_len   int   minimum accepted length (in Unicode code points)
max_len   int   maximum accepted length (in Unicode code points)

Example:

$options = [
    'options' => [
        'min_len' => 2,
        'max_len' => 2,
    ],
];

var_dump(
    filter_var('ab', FILTER_VALIDATE_STRLEN, $options),
    filter_var('🐘🐘', FILTER_VALIDATE_STRLEN, $options),
    filter_var('🐘', FILTER_VALIDATE_STRLEN, $options),
    filter_var('🐘🐘🐘', FILTER_VALIDATE_STRLEN, $options)
);

Implementation notes
The implementation:

• counts UTF-8 code points
• reuses UTF-8 handling logic already present in core (similar to the implementation used by htmlspecialchars)

Motivation

Validating the length of UTF-8 strings is a common requirement in applications (e.g. form input validation).

Currently this is typically implemented in userland using:

• mb_strlen() from the mbstring extension
• framework or CMS polyfills
• custom helpers

Providing a dedicated validator in the filter extension allows this validation to be performed in a lightweight and consistent way without introducing a dependency on mbstring.

Related discussion

See issue #21428