xml_set_external_entity_ref_handler()

The xml_set_external_entity_ref_handler() function is a PHP built-in function that sets a user-defined callback as the handler for external entity references in a legacy SAX XML parser. It is useful when you need to intercept and process external entities referenced in an XML document, such as loading external data from a database or performing custom validation.

Syntax

The syntax of the xml_set_external_entity_ref_handler() function is as follows:

syntax of the xml_set_external_entity_ref_handler() function in PHP

xml_set_external_entity_ref_handler($parser, $handler)

Where $parser is the XML parser on which the handler is set, and $handler is the name of the user-defined function that will handle external entity references.

Usage Examples

Let's take a look at a practical example of using xml_set_external_entity_ref_handler() in PHP.

Example: Setting an External Entity Reference Handler Function

Suppose you have an XML document that references external entities and you want to parse it using the legacy SAX parser in PHP. You can use the xml_parser_create() function to create a new XML parser, set an external entity reference handler using xml_set_external_entity_ref_handler(), and then parse the data:

Setting an External Entity Reference Handler Function in PHP

function handle_external_entity_ref($parser, $open_entity_names, $base, $system_id, $public_id) {
    // Process the external entity reference
    echo "External entity found: $system_id\n";
}

$xml_parser = xml_parser_create();
xml_set_external_entity_ref_handler($xml_parser, "handle_external_entity_ref");

// Parse XML data (the handler is invoked when an external entity is encountered)
$xml_data = '<!DOCTYPE foo [<!ENTITY xxe SYSTEM "http://example.com/data">]><foo/>';
xml_parse($xml_parser, $xml_data);
xml_parser_free($xml_parser);

⚠️ Security Warning: Handling external entities is a common vector for XML External Entity (XXE) attacks. Always validate and restrict external entity references, avoid loading local files, and consider using modern XML libraries (like DOMDocument with appropriate security flags) instead of the legacy SAX parser for security-sensitive applications.

Conclusion

In this article, we've discussed PHP's xml_set_external_entity_ref_handler() function and how it can be used to set an external entity reference handler for a legacy SAX XML parser. We've explained its syntax and provided a complete example showing how the callback is invoked during parsing. By using this function, you can intercept external entity references and perform custom actions, though you should always exercise caution to prevent XXE vulnerabilities.

Practice

What function in PHP is used to create an external entity reference for XML files?

xml_set_external_entity_ref_handler()xml_create_external_entity_ref()create_xml_entity_ref()xml_set_entity_ref()