先来看看下面这段代码:
<?php $arr = array('apple','banana','cat','dog'); foreach($arr as $key=>$val) { //some code } echo $val; //输出dog echo $key; //输出3 //下面对val进行赋值 $val = 'e'; print_r($arr); //输出Array ( [0] => apple [1] => banana [2] => cat [3] => dog )
说明:在上面的foreach循环中,当循环结束后,$key和$val变量都不会被自动释放掉。值会被保存下来。而且此时修改$val的值不会影响$arr。
引用:如果想在遍历数组的过程中修改数组的元素,可以在foreach中对$val使用引用。此时被引用的元素$val指向当前数组元素的内存地址,即共享一段内存地址。因此修改$val的值会同时改变$arr[$key]的值。
再来看看下面一段在foreach中使用引用的代码,这是最近在项目中遇到的一种情况:
<?php $arr = array('apple','banana','cat','dog'); //在foreach中使用引用 foreach($arr as $key => &$val) { $val = 'new value'; } echo $val; //输出new value echo $key; //输出3 $val = 'egg'; print_r($arr); //输出Array ( [0] => new value [1] => new value [2] => new value [3] => egg )
说明:在foreach中使用&引用后,当foreach结束后,$key和$val变量也都不会被自动释放掉,但是此时$val和$arrcount($arr) - 1指向相同的内存地址。因此,此时修改$val的值也会改变了$arr[3]的值。
这种情况下很容易犯的错误就是像上面例子所示,在循环外面继续使用被foreach引用的变量,这样会使开发者得不到预期的数据。因此,为了避免这种情况的发生,应该在适当的位置释放变量的引用。以上面的代码为例:
<?php $arr = array('apple','banana','cat','dog'); //在foreach中使用引用 foreach($arr as $key => &$val) { $val = 'new value'; } unset($val); echo $val; //报错,Notice: Undefined variable: val echo $key; //输出3 $val = 'egg'; print_r($arr); //输出Array ( [0] => new value [1] => new value [2] => new value [3] => new value ) 在foreach结束后unset $val,此时会释放对$val的引用。因此改变$val不会对$arr造成影响。
这是最近在项目中遇到的坑和解决方案的总结,如果错误或更好地建议,欢迎指出。

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