Include mbstring polyfill

[swissspidy]

Feature description

I originally opened this at GoogleForCreators/web-stories-wp#4898 but this might be useful as part of the AMP plugin itself.

amp-wp/lib/common/src/Dom/Document.php

Lines 1072 to 1076 in 708a65b

	if (self::AMP_ENCODING !== strtolower($this->originalEncoding)) {
	$target = function_exists('mb_convert_encoding')
	? mb_convert_encoding($source, self::AMP_ENCODING, $this->originalEncoding)
	: false;
	}

The Document class uses mb_convert_encoding to fix the encoding if needed. That function is part of the mbstring extension which can be missing or outdated on certain servers.

For those cases, https://github.com/symfony/polyfill-mbstring/ might be useful.

Caveats:

1. Might need to use PHP-Scoper to prefix the classes and prevent conflicts with other plugins using this polyfill
2. Might need to use mcaskill/composer-exclude-files to prevent files from being loaded automatically and steer that ourselves.

Curious to hear thoughts on this.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This is only necessary if a site is not using UTF-8 already, correct?

It looks like 95% of sites use UTF-8 already.

So probably very few sites would ever encounter an issue here?

[swissspidy]

Uhm, probably? Not sure how many sites have odd blog_charset configurations.

[westonruter]

Did it come up as an actual problem for Web Stories?

[swissspidy]

We had a couple of reports about mangled special chars in the past, where the encoding was off. But we'd also use mb_convert_encoding for other cases, such as GoogleForCreators/web-stories-wp#4859

[westonruter]

A question was raised in php-css-parser about whether mbstring should be made a hard-requirement: MyIntervals/PHP-CSS-Parser#254. Nevertheless, polyfill-mbstring requires PHP 7.1+ so we can't use it anyway.

[swissspidy]

We use an older version (1.18) which still supports PHP 5.6+. So does 1.19 I think (at least from what I can tell after looking at symfony/polyfill-mbstring@39d483b)