WP_User::__set( string $key, mixed $value )

Source

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

public function __set( $key, $value ) {
		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>'
				)
			);
			$this->ID = $value;
			return;
		}

		$this->data->$key = $value;
	}