<?php
# 值遍历
$arr = [0, 1, 2, 3, 4, 5];
$ref = &$arr;
foreach ($arr as $v) {
    if ($v === 0) {
        unset($arr[3]);
    }
    echo $v;
}
// output in 5.x: 01245
// output in 7.x: 012345  // 7.x版本下，通过值遍历时，底层操作的始终是数组的副本

# 通过引用来进行迭代 foreach ($arr as &$v)
$arr = [0, 1, 2, 3, 4, 5];
foreach ($arr as &$v) {
    if ($v === 0) {
        unset($arr[3]);
    }
    echo $v;
}
// output in 5.x: 01245
// output in 7.x: 01245

<?php
$data = ['a','b','c'];

foreach ($data as $k => &$v){
    //
    
}

var_dump($data);


foreach ($data as $k => $v){
    //
    
}

var_dump($data);