Upload: Return a translatable error instead of a fatal error when zlib is missing during PclZip extraction

[irozum]

What this fixes

When a PHP installation is missing both the ZipArchive class and the zlib extension (gzopen()), unzip_file() falls through to the bundled PclZip library, whose constructor calls die('Abort class-pclzip.php : Missing zlib extensions'). This is an untranslated, unstyled fatal error rather than the usual WP_Error that callers (like the plugin/theme upload screens) expect and render nicely.

The fix

• Added a check at the top of _unzip_file_pclzip() in src/wp-admin/includes/file.php that detects the missing zlib extension before ever instantiating PclZip, and returns a translatable WP_Error (unzip_file_missing_zlib) instead.
• The check is exposed via a new unzip_file_pclzip_zlib_available filter (mirroring the existing unzip_file_use_ziparchive filter in the same file), which also makes the new behavior directly unit-testable without needing an actual zlib-less PHP build.
• Left the vendored class-pclzip.php untouched, since it's third-party library code.

Testing

• Added test_should_return_wp_error_when_zlib_extension_is_unavailable() in tests/phpunit/tests/filesystem/unzipFilePclzip.php, which forces the new filter to false and asserts a WP_Error with the new error code is returned.
• Ran the full file test group, PHPCS, and PHPStan — all green.
• This bug only reproduces on PHP builds lacking zlib, which practically all modern installs have, so it isn't reproducible via manual UI steps in this repo's local dev environment; the added/passing test is the verification path.

Trac ticket: https://core.trac.wordpress.org/ticket/30963

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementation, tests, and PR description. Reviewed by Igor Rozum.

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