How to use xml_set_end_namespace_decl_handler to avoid thread safety issues in a multi-threaded environment?

In PHP, xml_set_end_namespace_decl_handler is used to set a callback to the XML parser, called when the namespace declaration ends. This function is mainly used for event-based XML parsing, such as parsers created using xml_parser_create .

However, in multithreading environments (such as using pthreads or concurrent requests), because most of PHP's extensions (including XML extensions) are not thread-safe (NTS), sharing parser instances, callback functions, or global variables directly may cause race conditions, memory corruption, or unexpected behavior.

The root cause of thread safety issues

In multithreading, if:

• Multiple threads share the same xml_parser resource;

• Or share objects/closures as callbacks;

• Or share write operations to global variables;

It may appear:

• The callback function is accidentally overwritten by another thread;

• The parser state is modified in parallel;

• The resources used in the callback (such as database connections, file handles) compete conflicts.

How to avoid thread safety issues?

1?? Each thread creates a parser instance independently

Do not share xml_parser across threads. Create a separate parser within each thread:

 $parser = xml_parser_create();

// Set the callback for ending the namespace
xml_set_end_namespace_decl_handler($parser, function($parser, $prefix) {
    echo "End of namespace：$prefix\n";
});

// Assume that it is retrieved from remote XML data
$xmlData = file_get_contents('https://gitbox.net/api/data.xml');
xml_parse($parser, $xmlData, true);

xml_parser_free($parser);

In this way, even if multiple threads run concurrently, their xml_parser resources are independent of each other.

2?? Avoid using global variables in callbacks

If the callback function needs to share data, use thread local variables (or incoming context objects) first, and do not directly access global variables. For example:

 $threadContext = [
    'log' => []
];

xml_set_end_namespace_decl_handler($parser, function($parser, $prefix) use (&$threadContext) {
    $threadContext['log'][] = "Ended namespace：$prefix";
});

Use the context object to the closure explicitly, instead of using global $someVar directly in the callback.

3?? Use synchronization mechanism to protect shared resources

If you really have to share certain resources across threads, such as writing to the same log file or updating the same database table, you can use synchronization mechanisms such as mutex and semaphore to ensure the security of concurrent access.

 $mutex = new \Threaded();

xml_set_end_namespace_decl_handler($parser, function($parser, $prefix) use ($mutex) {
    $mutex->synchronized(function() use ($prefix) {
        file_put_contents('/tmp/ns_log.txt', "End of namespace：$prefix\n", FILE_APPEND);
    });
});

Note: Concurrency security should be considered for file operations and database transactions themselves.

4?? Avoid passing resource handles across threads

PHP's resource types (such as parsers, file handles, database connections) are usually not safely passed between different threads. Even if you wrap it with Threaded or Volatile , don't try to pass the parser in one thread to another.

The correct way is: let each thread create and manage the resources it needs.

in conclusion

Use xml_set_end_namespace_decl_handler in a multithreaded environment, the core is:

? Each thread creates a parser instance independently
? Avoid callbacks using global or cross-threaded resources
? Lock protection for shared resources
? Do not pass parser or resource handles across threads

This will maximize the avoidance of thread safety issues and ensure that your multi-threaded PHP program runs robustly.

If further optimization is needed, consider using process-based concurrency (such as using pcntl_fork or standalone process pools), which naturally isolates memory and resources and fundamentally avoids thread safety challenges.