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https://www.php.net/manual/kr/keyword.extends.php is broken. Found link through google search.
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The keywords.extends.php page was removed from the manual back in November 2016. The page was part of the PHP 4 manual, which was removed at this time.
The correct manual page for the extends keyword, in PHP 5 onwards, is the "Classes and Objects: The Basics" chapter of the manual at https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.extends
On a related note, the Korean translation of the manual has not been maintained for a very long time and officially marked as inactive in July 2016.
Related To: Bug #77839