libxml Functions
PHP Manual

libxml_set_external_entity_loader

Changes the default external entity loader

Description

bool libxml_set_external_entity_loader(callablenull $resolver_function)

Changes the default external entity loader. This can be used to suppress the expansion of arbitrary external entities to avoid XXE attacks, even when LIBXML_NOENT has been set for the respective operation, and is usually preferable over calling libxml_disable_entity_loader.

Parameters

resolver_function

A callable with the following signature:

resourcestringnull resolver(stringnull $public_id, string $system_id, array $context)
public_id
The public ID.
system_id
The system ID.
context
An array with the four elements "directory", "intSubName", "extSubURI" and "extSubSystem".
This callable should return a resource, a string from which a resource can be opened. If null is returned, the entity reference resolution will fail.

Return Values

Returns true on success or false on failure.

Examples

Example #1 libxml_set_external_entity_loader example

<?php
$xml = <<<XML
<!DOCTYPE foo PUBLIC "-//FOO/BAR" "http://example.com/foobar">
<foo>bar</foo>
XML;

$dtd = <<<DTD
<!ELEMENT foo (#PCDATA)>
DTD;

libxml_set_external_entity_loader(
    function ($public, $system, $context) use($dtd) {
        var_dump($public);
        var_dump($system);
        var_dump($context);
        $f = fopen("php://temp", "r+");
        fwrite($f, $dtd);
        rewind($f);
        return $f;
    }
);

$dd = new DOMDocument;
$r  = $dd->loadXML($xml);

var_dump($dd->validate());
?>

The above example will output:

string(10) "-//FOO/BAR"
string(25) "http://example.com/foobar"
array(4) {
    ["directory"]    => NULL
    ["intSubName"]   => NULL
    ["extSubURI"]    => NULL
    ["extSubSystem"] => NULL
}
bool(true)

See Also

  • libxml_disable_entity_loader
  • libxml_get_external_entity_loader