Alphabetical order for variation dropdowns

variation_attribute_options_html.php

<?php
add_filter('woocommerce_dropdown_variation_attribute_options_html', function ($html) {
	$xml = simplexml_load_string($html);

	$select = array('id'               => (string)$xml->attributes()['id'],
	                'name'             => (string)$xml->attributes()['name'],
	                'attribute_name'   => (string)$xml->attributes()['data-attribute_name'],
	                'show_option_none' => (string)$xml->attributes()['data-show_option_none']);

	$options = array();
	foreach($xml->option as $option) {
		$options[] = array('text'     => (string)$option,
		                   'value'    => (string)$option->attributes()['value'],
		                   'selected' => (string)$option->attributes()['selected'] === 'selected');
	}

	usort($options, function ($a, $b) {
		return strcmp($a['value'], $b['value']);
	});

	$xml = simplexml_load_string('<select/>');
	$xml->addAttribute("id", $select["id"]);
	$xml->addAttribute("name", $select["name"]);
	$xml->addAttribute("data-attribute_name", $select["attribute_name"]);
	$xml->addAttribute("data-show_option_none", $select["show_option_none"]);

	foreach ($options as $option) {
		$child = $xml->addChild('option');
		$child->value = $option['text'];
		$child->addAttribute('value', $option['value']);
		if ($option['selected']) {
			$child->addAttribute('selected', 'selected');
		}
	}

	return $xml->asXML();
});