foreach的奇怪行为

这是有据可查的PHP行为，请参阅php.net的foreach页面上的警告。

警告

即使在 foreach 循环之后，仍保留 $ value的 引用和最后一个数组元素。建议通过unset（）销毁它。 __

$a = array('a', 'b', 'c', 'd');foreach ($a as &$v) { }unset($v);foreach ($a as $v) { }print_r($a);

编辑

尝试逐步了解此处实际发生的情况

$a = array('a', 'b', 'c', 'd');foreach ($a as &$v) { }   // 1st iteration $v is a reference to $a[0] ('a')foreach ($a as &$v) { }   // 2nd iteration $v is a reference to $a[1] ('b')foreach ($a as &$v) { }   // 3rd iteration $v is a reference to $a[2] ('c')foreach ($a as &$v) { }   // 4th iteration $v is a reference to $a[3] ('d')    // At the end of the foreach loop,    //    $v is still a reference to $a[3] ('d')foreach ($a as $v) { }    // 1st iteration $v (still a reference to $a[3])     //    is set to a value of $a[0] ('a').    //    Because it is a reference to $a[3],     //    it sets $a[3] to 'a'.foreach ($a as $v) { }    // 2nd iteration $v (still a reference to $a[3])     //    is set to a value of $a[1] ('b').    //    Because it is a reference to $a[3],     //    it sets $a[3] to 'b'.foreach ($a as $v) { }    // 3rd iteration $v (still a reference to $a[3])     //    is set to a value of $a[2] ('c').    //    Because it is a reference to $a[3],     //    it sets $a[3] to 'c'.foreach ($a as $v) { }    // 4th iteration $v (still a reference to $a[3])     //    is set to a value of $a[3] ('c' since     //       the last iteration).    //    Because it is a reference to $a[3],     //    it sets $a[3] to 'c'.