They are visibilty identifiers
:
Visibility is similar to variable scope, however, offers finer control. There are three types of visibility.

• Public (default) - the variable can be accessed and changed globally by anything.

• Protected - the variable can be accessed and changed only by direct descendants (those who inherit it using the extends statement) of the class and class its self

• Private - the variable can be accessed and changed only from within the class.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)