Edit: https-detection.php

<?php
/**
 * HTTPS detection functions.
 *
 * @package WordPress
 * @since 5.7.0
 */

/**
 * Checks whether the website is using HTTPS.
 *
 * This is based on whether both the home and site URL are using HTTPS.
 *
 * @since 5.7.0
 * @see wp_is_home_url_using_https()
 * @see wp_is_site_url_using_https()
 *
 * @return bool True if using HTTPS, false otherwise.
 */
function wp_is_using_https() {
	if ( ! wp_is_home_url_using_https() ) {
		return false;
	}

return wp_is_site_url_using_https();
}

/**
 * Checks whether the current site URL is using HTTPS.
 *
 * @since 5.7.0
 * @see home_url()
 *
 * @return bool True if using HTTPS, false otherwise.
 */
function wp_is_home_url_using_https() {
	return 'https' === wp_parse_url( home_url(), PHP_URL_SCHEME );
}

/**
 * Checks whether the current site's URL where WordPress is stored is using HTTPS.
 *
 * This checks the URL where WordPress application files (e.g. wp-blog-header.php or the wp-admin/ folder)
 * are accessible.
 *
 * @since 5.7.0
 * @see site_url()
 *
 * @return bool True if using HTTPS, false otherwise.
 */
function wp_is_site_url_using_https() {
	/*
	 * Use direct option access for 'siteurl' and manually run the 'site_url'
	 * filter because `site_url()` will adjust the scheme based on what the
	 * current request is using.
	 */
	/** This filter is documented in wp-includes/link-template.php */
	$site_url = apply_filters( 'site_url', get_option( 'siteurl' ), '', null, null );

return 'https' === wp_parse_url( $site_url, PHP_URL_SCHEME );
}

/**
 * Checks whether HTTPS is supported for the server and domain.
 *
 * This function makes an HTTP request through `wp_get_https_detection_errors()`
 * to check for HTTPS support. As this process can be resource-intensive,
 * it should be used cautiously, especially in performance-sensitive environments,
 * to avoid potential latency issues.
 *
 * @since 5.7.0
 *
 * @return bool True if HTTPS is supported, false otherwise.
 */
function wp_is_https_supported() {
	$https_detection_errors = wp_get_https_detection_errors();

// If there are errors, HTTPS is not supported.
	return empty( $https_detection_errors );
}

/**
 * Runs a remote HTTPS request to detect whether HTTPS supported, and stores potential errors.
 *
 * This function checks for HTTPS support by making an HTTP request. As this process can be resource-intensive,
 * it should be used cautiously, especially in performance-sensitive environments.
 * It is called when HTTPS support needs to be validated.
 *
 * @since 6.4.0
 * @access private
 *
 * @return array An array containing potential detection errors related to HTTPS, or an empty array if no errors are found.
 */
function wp_get_https_detection_errors() {
	/**
	 * Short-circuits the process of detecting errors related to HTTPS support.
	 *
	 * Returning a `WP_Error` from the filter will effectively short-circuit the default logic of trying a remote
	 * request to the site over HTTPS, storing the errors array from the returned `WP_Error` instead.
	 *
	 * @since 6.4.0
	 *
	 * @param null|WP_Error $pre Error object to short-circuit detection,
	 *                           or null to continue with the default behavior.
	 */
	$support_errors = apply_filters( 'pre_wp_get_https_detection_errors', null );
	if ( is_wp_error( $support_errors ) ) {
		return $support_errors->errors;
	}

$support_errors = new WP_Error();

$response = wp_remote_request(
		home_url( '/', 'https' ),
		array(
			'headers'   => array(
				'Cache-Control' => 'no-cache',
			),
			'sslverify' => true,
		)
	);

if ( is_wp_error( $response ) ) {
		$unverified_response = wp_remote_request(
			home_url( '/', 'https' ),
			array(
				'headers'   => array(
					'Cache-Control' => 'no-cache',
				),
				'sslverify' => false,
			)
		);

if ( is_wp_error( $unverified_response ) ) {
			$support_errors->add(
				'https_request_failed',
				__( 'HTTPS request failed.' )
			);
		} else {
			$support_errors->add(
				'ssl_verification_failed',
				__( 'SSL verification failed.' )
			);
		}

$response = $unverified_response;
	}

if ( ! is_wp_error( $response ) ) {
		if ( 200 !== wp_remote_retrieve_response_code( $response ) ) {
			$support_errors->add( 'bad_response_code', wp_remote_retrieve_response_message( $response ) );
		} elseif ( false === wp_is_local_html_output( wp_remote_retrieve_body( $response ) ) ) {
			$support_errors->add( 'bad_response_source', __( 'It looks like the response did not come from this site.' ) );
		}
	}

return $support_errors->errors;
}

/**
 * Checks whether a given HTML string is likely an output from this WordPress site.
 *
 * This function attempts to check for various common WordPress patterns whether they are included in the HTML string.
 * Since any of these actions may be disabled through third-party code, this function may also return null to indicate
 * that it was not possible to determine ownership.
 *
 * @since 5.7.0
 * @access private
 *
 * @param string $html Full HTML output string, e.g. from a HTTP response.
 * @return bool|null True/false for whether HTML was generated by this site, null if unable to determine.
 */
function wp_is_local_html_output( $html ) {
	// 1. Check if HTML includes the site's Really Simple Discovery link.
	if ( has_action( 'wp_head', 'rsd_link' ) ) {
		$pattern = preg_replace( '#^https?:(?=//)#', '', esc_url( site_url( 'xmlrpc.php?rsd', 'rpc' ) ) ); // See rsd_link().
		return str_contains( $html, $pattern );
	}

// 2. Check if HTML includes the site's REST API link.
	if ( has_action( 'wp_head', 'rest_output_link_wp_head' ) ) {
		// Try both HTTPS and HTTP since the URL depends on context.
		$pattern = preg_replace( '#^https?:(?=//)#', '', esc_url( get_rest_url() ) ); // See rest_output_link_wp_head().
		return str_contains( $html, $pattern );
	}

// Otherwise the result cannot be determined.
	return null;
}

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