> […] but I assume there is some reason why this is not feasible

See bug #72793.

Poked this a little farther and determined there is some more nuance than I originally understood.  Here is the code to reproduce my case (can't seem to update the script field via the edit interface)

<?php

class c
{
	private $xml;
	private $test;

	public function test()
	{
		$this->xml = xml_parser_create();
		xml_set_character_data_handler($this->xml, array(&$this, 'handle_cdata'));
		xml_parser_free($this->xml);

		$this->test = str_repeat('xxxxx', 1000000);
	}

	public function test2()
	{
		$xml = xml_parser_create();
		xml_set_character_data_handler($xml, array(&$this, 'handle_cdata'));
		xml_parser_free($xml);

		$this->test = str_repeat('xxxxx', 1000000);
	}

	public function test3()
	{
		$this->xml = xml_parser_create();
		xml_set_character_data_handler($this->xml, array(&$this, 'handle_cdata'));
		xml_set_character_data_handler($this->xml, null);
		xml_parser_free($this->xml);

		$this->test = str_repeat('xxxxx', 1000000);
	}

	public function handle_cdata(&$parser, $data)
	{
	}
}

for($i = 1; $i < 100; $i++)
{
	$object = new c();
	$object->test();
	unset($object);
	var_dump(memory_get_usage(true) / (1024*1024));
}
?>

Note that test will show the memory leak, but test2 and test3 don't.  I don't know if that's the same as bug #72793 or not.

Good catch, @ksours!  This is not exactly the other bug, but it
seems it has the same root cause.  If an object references a
resource, and this resource references that object, there is a
cycle that could only be resolved by the cyclic GC, but since
resources are not tracked (and that's unlikely to happen in the
future), it won't.  The proper solution would be to turn the
parser resource into an object.

The docs should be okay now, so changing to the actual bug.

WIP patch up at https://github.com/php/php-src/pull/3526.

ext/xml has been migrated to use objects in PHP 8, so the issue will be resolved there.