Source

File: wp-admin/includes/class-wp-site-health.php

function can_perform_loopback() {
		$cookies = wp_unslash( $_COOKIE );
		$timeout = 10;
		$headers = array(
			'Cache-Control' => 'no-cache',
		);
		/** This filter is documented in wp-includes/class-wp-http-streams.php */
		$sslverify = apply_filters( 'https_local_ssl_verify', false );

		// Include Basic auth in loopback requests.
		if ( isset( $_SERVER['PHP_AUTH_USER'] ) && isset( $_SERVER['PHP_AUTH_PW'] ) ) {
			$headers['Authorization'] = 'Basic ' . base64_encode( wp_unslash( $_SERVER['PHP_AUTH_USER'] ) . ':' . wp_unslash( $_SERVER['PHP_AUTH_PW'] ) );
		}

		$url = admin_url();

		$r = wp_remote_get( $url, compact( 'cookies', 'headers', 'timeout', 'sslverify' ) );

		if ( is_wp_error( $r ) ) {
			return (object) array(
				'status'  => 'critical',
				'message' => sprintf(
					'%s<br>%s',
					__( 'The loopback request to your site failed, this means features relying on them are not currently working as expected.' ),
					sprintf(
						/* translators: 1: The WordPress error message. 2: The WordPress error code. */
						__( 'Error: %1$s (%2$s)' ),
						$r->get_error_message(),
						$r->get_error_code()
					)
				),
			);
		}

		if ( 200 !== wp_remote_retrieve_response_code( $r ) ) {
			return (object) array(
				'status'  => 'recommended',
				'message' => sprintf(
					/* translators: %d: The HTTP response code returned. */
					__( 'The loopback request returned an unexpected http status code, %d, it was not possible to determine if this will prevent features from working as expected.' ),
					wp_remote_retrieve_response_code( $r )
				),
			);
		}

		return (object) array(
			'status'  => 'good',
			'message' => __( 'The loopback request to your site completed successfully.' ),
		);
	}