auto-extract-alt-text.php

<?php
/**
 * Extract alt meta data from uploaded images.
 *
 * @wordpress-plugin
 * Plugin Name: Extract Alt on Upload
 * Description:  Extract alt meta data from uploaded images.
 * Plugin URI:
 * Version:     1.0.0
 * Author:      Adam Silverstein.
 * License:     GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */
 
 function filter_wp_read_image_metadata( $meta, $file ) {
	$jpeg_contents = file_get_contents( $file );
	$value = false;
	// Find the start and end positions of the XMP metadata
	$xmp_start = strpos( $jpeg_contents, '<x:xmpmeta' );
	$xmp_end = strpos( $jpeg_contents, '</x:xmpmeta>');

	// Extract the XMP metadata from the JPEG contents
	$xmp_data = substr( $jpeg_contents, $xmp_start, $xmp_end - $xmp_start + 12 );

	// Parse the XMP metadata using DOMDocument
	$doc = new DOMDocument();
	$doc->loadXML( $xmp_data );

	// Instantiate an XPath object, used to extract portions of the XMP.
	$xpath = new DOMXPath( $doc );

	// Register the relevant XML namespaces.
	$xpath->registerNamespace( 'x', 'adobe:ns:meta/' );
	$xpath->registerNamespace( 'rdf', 'http://www.w3.org/1999/02/22-rdf-syntax-ns#' );
	$xpath->registerNamespace( 'Iptc4xmpCore', 'http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/' );

	$node_list = $xpath->query( '/x:xmpmeta/rdf:RDF/rdf:Description/Iptc4xmpCore:AltTextAccessibility' );
	if ( $node_list && $node_list->count() ) {
		// Get the alt text accessibility alternative most appropriate for the site language.

		$node = $node_list->item( 0 );

		// Get the site's locale.
		$locale = get_locale();

		// There are 3 possibilities:
		//
		// 1. there is an rdf:li with an exact match on the site locale
		// 2. there is an rdf:li with a partial match on the site locale (e.g., site locale is en_US and rdf:li has @xml:lang="en")
		// 3. there is an rdf:li with an "x-default" lang.
		//
		// we evaluate them in that order, stopping when we have a match.
		$value = $xpath->evaluate( "string( rdf:Alt/rdf:li[ @xml:lang = '{$locale}' ] )", $node );
		if ( ! $value ) {
				$value = $xpath->evaluate( 'string( rdf:Alt/rdf:li[ @xml:lang = "' . substr( $locale, 0, 2 ) . '" ] )', $node );
				if ( ! $value ) {
						$value = $xpath->evaluate( 'string( rdf:Alt/rdf:li[ @xml:lang = "x-default" ] )', $node );
				}
		}
	}
	if ( $value ) {
		$callback = function( $attachment_id ) use ( &$callback, $value ) {
			$attachment = get_post( $attachment_id );
			if ( ! wp_attachment_is_image( $attachment_id ) ) {
				return;
			}
			update_post_meta( $attachment_id, '_wp_attachment_image_alt', $value );
			remove_action( 'add_attachment', $callback );
		};
		add_action( 'add_attachment', $callback );
	}

	return $meta;
}
add_filter( 'wp_read_image_metadata', 'filter_wp_read_image_metadata', 10, 2 );

Hey Adam this plugin is so useful. Installed it a week ago and it worked right out of the box. Strangely a week of site updates later it is still enabled and activated but does not work. Tried reinstalling and even copying your code to a new custom plugin... no dice. Currently on WP 6.8.2.

sorry to hear it broke, have you tried testing with the previous images? I don't know if its robust enough to work with every image.

I have installed the plugin "Enable Replace Media". Uploading a new image file adds the alt-text correctly. However when I replace the media the alt-text field gets cleared. Would you happen to know how to fix this? Updating the alt-text when replacing an image would be very helpful!

Hey @hlsautter - I assume you are talking about https://wordpress.org/plugins/enable-media-replace/? The gist relies on the wp_read_image_metadata so perhaps that plugin removes it or overwrites what the gist does. You could try changing to a later priority:

change the last line to -

add_filter( 'wp_read_image_metadata', 'filter_wp_read_image_metadata', 99, 2 );

I tried this update and got the same results. - load the original IPTC file and the alt text was available as expected - edited the alt text value - uploaded the original file again using “replace” media - the Alt text was not updated from the file (expected the original alt-text instead of the edited value)

…

On Oct 16, 2025, at 1:44 PM, Adam Silverstein ***@***.***> wrote: @adamsilverstein commented on this gist. Hey @hlsautter <https://github.com/hlsautter> - I assume you are talking about https://wordpress.org/plugins/enable-media-replace/? The gist relies on the wp_read_image_metadata so perhaps that plugin removes it or overwrites what the gist does. You could try changing to a later priority: change the last line to - add_filter( 'wp_read_image_metadata', 'filter_wp_read_image_metadata', 99, 2 ); — Reply to this email directly, view it on GitHub <https://gist.github.com/adamsilverstein/e9c9993d938eae80e559d9eda8c0fdcc#gistcomment-5808810> or unsubscribe <https://github.com/notifications/unsubscribe-auth/APA3CY7APGPVTS3WKIELF6L3X77S5BFHORZGSZ3HMVZKMY3SMVQXIZNMON2WE2TFMN2F65DZOBS2WR3JON2EG33NNVSW45FGORXXA2LDOOIYFJDUPFYGLJDHNFZXJJLWMFWHKZNJGE2DAOBWGIYTMNFKMF2HI4TJMJ2XIZLTSOBKK5TBNR2WLJZSGY3TMMBSGKSG4YLNMWUGCY3UN5ZF62LEQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZQKSXMYLMOVS2IZ3JON2KI3TBNVS2W5DIOJSWCZC7OR4XAZI>. You are receiving this email because you were mentioned. Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.

I would need to look at the plugin to see why its not quite compatible with this code. Hopefully this will land in core so you won't need my plugin.