Prevent fatal error in wp_get_image_alttext() when DOM extension is missing

[robelsust]

Trac ticket: https://core.trac.wordpress.org/ticket/66221

When uploading an image (or when reading image metadata via wp_read_image_metadata()), wp_get_image_alttext() is invoked to extract alternative text embedded in XMP metadata.

Currently, wp_get_image_alttext() instantiates new DOMDocument() and new DOMXPath() without verifying if PHP's dom extension (ext-dom) is installed or available on the host environment:

Fatal error: Uncaught Error: Class "DOMDocument" not found in .../wp-admin/includes/image.php

While PHP's dom extension is strongly recommended by WordPress hosting requirements, it is not strictly required across all minimal hosting environments. WordPress core provides guards in other areas (such as iis7_rewrite_rule_exists(), wp_oembed_get(), and WP_Widget_Text) using class_exists( 'DOMDocument', false ).

Changes Proposed

1. Guard wp_get_image_alttext() against missing DOMDocument or DOMXPath classes early at the beginning of the function:

   if ( ! class_exists( 'DOMDocument', false ) || ! class_exists( 'DOMXPath', false ) ) {
       return $alt_text;
   }

   Checking at the start of the function rather than immediately before DOM instantiation also avoids reading large image files into memory via file_get_contents() and running string searches when the metadata cannot be parsed.
2. Add @covers ::wp_get_image_alttext and unit tests in tests/phpunit/tests/image/meta.php:
   • test_wp_get_image_alttext(): Tests extraction with a valid IPTC/XMP test image.
   • test_wp_get_image_alttext_without_xmp(): Tests fallback with an image containing no XMP metadata.

---

Testing Instructions

1. Run PHPUnit test suite:

   vendor/bin/phpunit tests/phpunit/tests/image/meta.php

2. Verify all assertions pass.
3. On an environment without ext-dom, verify that uploading an image or calling wp_read_image_metadata() completes without a fatal error.

---

AI Disclosure: Code changes and documentation were prepared with AI assistance, verified and tested manually against WordPress Core coding standards and PHPUnit test suite.

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[aaronjorbin]

For the tests, it would be best to be thorough. The code notes that there are three possibilities for alt text, so each of those should get tested. Additionally, the tests will return different alt text based on the site local, that functionality should be tested.

[aaronjorbin]

Suggested change

	* @return string Embedded alternative text.
	* @return string Embedded alternative text, empty when there is no alt text or DOM extension is not installed.

I think the empty string should get noted.

[aaronjorbin]

These should go in their own test file since they are testing something else. tests/phpunit/tests/image/alttext.php feels like is a good choice

[aaronjorbin]

Since this will return an empty string when Dom is not available, it should be properly handled in the test as well.

[robelsust]

Thanks for the thorough review and guidance, @aaronjorbin!

I have addressed all the feedback:

1. DocBlock: Updated the @return tag description for wp_get_image_alttext() to note that an empty string is returned when alt text is absent or the DOM extension is not installed.
2. Dedicated Test File: Moved the tests out of meta.php into a dedicated test class at tests/phpunit/tests/image/alttext.php.
3. Thorough Test Coverage:
   • Possibility 1: Exact match on site locale (e.g. de_DE).
   • Possibility 2: Partial match on site locale (e.g. es_ES matching xml:lang="es").
   • Possibility 3: Fallback to x-default when neither exact nor partial locale matches.
   • Tested that switching site locales returns the corresponding localized alt text.
   • Tested edge cases (missing XMP, missing AltTextAccessibility node).
4. DOM Availability: All tests verify the extracted text when DOM is present, while gracefully expecting an empty string if the DOM extension is not installed on the test environment.