WP_Http_Streams::verify_ssl_certificate() WordPress Method

	public static function verify_ssl_certificate( $stream, $host ) {
		$context_options = stream_context_get_options( $stream );

		if ( empty( $context_options['ssl']['peer_certificate'] ) ) {
			return false;
		}

		$cert = openssl_x509_parse( $context_options['ssl']['peer_certificate'] );
		if ( ! $cert ) {
			return false;
		}

		/*
		 * If the request is being made to an IP address, we'll validate against IP fields
		 * in the cert (if they exist)
		 */
		$host_type = ( WP_Http::is_ip_address( $host ) ? 'ip' : 'dns' );

		$certificate_hostnames = array();
		if ( ! empty( $cert['extensions']['subjectAltName'] ) ) {
			$match_against = preg_split( '/,\s*/', $cert['extensions']['subjectAltName'] );
			foreach ( $match_against as $match ) {
				list( $match_type, $match_host ) = explode( ':', $match );
				if ( strtolower( trim( $match_type ) ) === $host_type ) { // IP: or DNS:
					$certificate_hostnames[] = strtolower( trim( $match_host ) );
				}
			}
		} elseif ( ! empty( $cert['subject']['CN'] ) ) {
			// Only use the CN when the certificate includes no subjectAltName extension.
			$certificate_hostnames[] = strtolower( $cert['subject']['CN'] );
		}

		// Exact hostname/IP matches.
		if ( in_array( strtolower( $host ), $certificate_hostnames, true ) ) {
			return true;
		}

		// IP's can't be wildcards, Stop processing.
		if ( 'ip' === $host_type ) {
			return false;
		}

		// Test to see if the domain is at least 2 deep for wildcard support.
		if ( substr_count( $host, '.' ) < 2 ) {
			return false;
		}

		// Wildcard subdomains certs (*.example.com) are valid for a.example.com but not a.b.example.com.
		$wildcard_host = preg_replace( '/^[^.]+\./', '*.', $host );

		return in_array( strtolower( $wildcard_host ), $certificate_hostnames, true );
	}