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unset()の前にisset()する必要はない

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未定義変数をいきなり使わないよう、isset()でチェックする心がけは良い習慣ですが
unset()する場合はisset()でチェックしなくてもUndefined VariableのNoticeにはなりません。

unset.php
<?php
error_reporting(E_ALL);
echo $foo;   //未定義の $fooを参照
unset($foo); //未定義の $fooをいきなりunset
echo 'Done.';

実行結果

$ php unset.php
PHP Notice:  Undefined variable: foo in /path/to/test.php on line 3  # echo はNoticeがでる
Done.            # unset は Noticeでない

おまけ:unset()は複数引数とれるので、まとめて処理できます。

<?php
unset($foo, $bar, $baz);
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