Source

File: wp-includes/class-wp-user.php

public function __get( $key ) {
		if ( 'id' === $key ) {
			_deprecated_argument(
				'WP_User->id',
				'2.1.0',
				sprintf(
					/* translators: %s: WP_User->ID */
					__( 'Use %s instead.' ),
					'<code>WP_User->ID</code>'
				)
			);
			return $this->ID;
		}

		if ( isset( $this->data->$key ) ) {
			$value = $this->data->$key;
		} else {
			if ( isset( self::$back_compat_keys[ $key ] ) ) {
				$key = self::$back_compat_keys[ $key ];
			}
			$value = get_user_meta( $this->ID, $key, true );
		}

		if ( $this->filter ) {
			$value = sanitize_user_field( $key, $value, $this->ID, $this->filter );
		}

		return $value;
	}