Error out when xml_create_parser is not available

[kraftbj]

When PHP XML is not available, we fatal with an PHP Fatal error: Call to undefined function xml_parser_create() in wp-includes/class-IXR.php and other files.

We already error out when that function is not available in wp-includes/rss.php since [3083].

Background: The PHP 7 packages on Ubuntu, unlike PHP 5, does not include php7.0-xml by default, so there may be an increase in failures of this type as folks switch over.

[kraftbj]

We use that function in the following files, seemingly without a check at first glance:

• /wp-admin/link-parse-opml.php (long live Links)
• /wp-includes/atomlib.php
• /wp-includes/class-simplepie.php
• /wp-includes/feed.php
• /wp-includes/SimplePie/Parser.php

and a couple of unit tests.

[kraftbj]

Check for xml_parser_create before use

[kraftbj]

Attached a patch. SimplePie appears to check for the xml extension directly before running, which likely covers it.

[jorbin]

Nice catch @kraftbj and agree that this makes sense.

[GaryJ]

The check before the solitary call to xml_parser_create_ns() is a check for xml_parser_create(). While presumably the former will be available when the latter is also available, why not actually check for the function that's about to be called?

For the cases when duplicate code is created (check + call to xml_parser_create()), I think it may make more sense to add in a wp_xml_parser_create() that can handle this in a cleaner way. i.e.

function wp_xml_parser_create( $encoding = null ) {
    if ( ! function_exists( 'xml_parser_create' ) ) { 
        trigger_error( __( 'The XML extension for PHP is not available. Please contact your hosting provider to enable it." ) ); 
    }

    return xml_parser_create( $encoding );
}

Then use wp_xml_parser_create() in the instances where non-library code in WP currently uses xml_parser_create().

Regardless of my suggestion, I think the patch needs a refresh, as some files have been moved around.

[kraftbj]

Good call wrt xml_parser_create_ns. I'm neutral on the helper function. I'm about to hit the road, but can refresh it later this week.

[markoheijnen]

I have uploaded a new patch that fixes the check for xml_parser_create_ns but mainly fixed the problem with previous code and first patch is still you still need to return something otherwise it's still fatal errors.

[stevenkword]

@kraftbj to review the most recent patches.

[kraftbj]

updated patch

[kraftbj]

I updated the patch some:

• src/wp-admin/link-parse-opml.php -- I kill processing using wp_die instead of the if/else structure. This file isn't called by Core, seemingly only called by the OPML Importer plugins (perhaps others).
• src/wp-includes/feed.php - Confirming that the given return makes enough sense. We aren't able to confirm it is XML, so returning as a looser type should be fine. prep_atom_text_construct() doesn't seem to be called by Core either.
• src/wp-includes/rss.php - I updated the error messages to be consistent with the others (and the php.net link isn't helpful for the PHP 7 issue). Previously, it called the parser while suppressing errors, then checking if it is a php resource. Changed it to be consistent with the other checks.

[markoheijnen]

Looks good to me but haven't been able to test it on my end as this fixes edge cases (removed functionality)

[pento]

In 38883:

General: Check to see that the PHP-XML module is enabled before using XML functions.

There are a handful of places where we don't check that the XML functions exist before we use them. Ubuntu's PHP 7 packages don't include PHP-XML by default, increasing the chance of this causing issues.

Props kraftbj, markoheijnen.
Fixes #37122.

[ocean90]

In 39007:

XML-RPC: Fix truncated warning message added in [38883].

See #37122.

[t0nnyh0ang]

Replying to kraftbj:

When PHP XML is not available, we fatal with an PHP Fatal error: Call to undefined function xml_parser_create() in wp-includes/class-IXR.php and other files.

We already error out when that function is not available in wp-includes/rss.php since [3083].

Background: The PHP 7 packages on Ubuntu, unlike PHP 5, does not include php7.0-xml by default, so there may be an increase in failures of this type as folks switch over.