php中双冒号的应用

<?php ????class?Fruit ?? { ??????static?function?color() ?? ????{ ??????????return?"color"; ?? ????} ????????static?function?showColor() ?? ????{ ??????????echo?"show?"?.?self::color(); ?? ????} ??} ????class?Apple?extends?Fruit ?? { ??????static?function?color() ?? ????{ ??????????return?"red"; ?? ????} ??} ????Apple::showColor(); ??//?output?is?"show?color"! ?? ???>??

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程序运行结果：
show color