关于数组的一点困惑解决方法

关于数组的一点困惑

PHP code

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$arr = array();
$num='hello world';

if(isset($num)) {
    $arr = $num;
    var_dump($arr);
} else {
    $arr[] = array(1,2,3);
    var_dump($arr);
}

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HTML code

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-->string(11) "hello world" 

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