What is foreach in php process control
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(PHP 4, PHP 5, PHP 7)
foreach 语法结构提供了遍历数组的简单方式。foreach 仅能够应用于数组和对象,如果尝试应用于其他数据类型的变量,或者未初始化的变量将发出错误信息。有两种语法:
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第一种格式遍历给定的 array_expression 数组。每次循环中,当前单元的值被赋给 $value 并且数组内部的指针向前移一步(因此下一次循环中将会得到下一个单元)。
第二种格式做同样的事,只除了当前单元的键名也会在每次循环中被赋给变量 $key。
还能够自定义遍历对象。
Note:
当 foreach 开始执行时,数组内部的指针会自动指向第一个单元。这意味着不需要在 foreach 循环之前调用 reset()。
由于 foreach 依赖内部数组指针,在循环中修改其值将可能导致意外的行为。
可以很容易地通过在 $value 之前加上 & 来修改数组的元素。此方法将以引用赋值而不是拷贝一个值。
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$value 的引用仅在被遍历的数组可以被引用时才可用(例如是个变量)。以下代码则无法运行:
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Warning
数组最后一个元素的 $value 引用在 foreach 循环之后仍会保留。建议使用 unset() 来将其销毁。
Note:
foreach 不支持用“@”来抑制错误信息的能力。
用户可能注意到了以下的代码功能完全相同:
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以下代码功能也完全相同:
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示范用法的更多例子:
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用 list() 给嵌套的数组解包 ¶
(PHP 5 >= 5.5.0, PHP 7)
PHP 5.5 增添了遍历一个数组的数组的功能并且把嵌套的数组解包到循环变量中,只需将 list() 作为值提供。
例如:
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以上例程会输出:
A: 1; B: 2 A: 3; B: 4
list() 中的单元可以少于嵌套数组的,此时多出来的数组单元将被忽略:
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以上例程会输出:
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如果 list() 中列出的单元多于嵌套数组则会发出一条消息级别的错误信息:
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以上例程会输出:
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