$arr = ['a', 'b', 'c'];
foreach ($arr as $key => &$value) {
    $value = $value . '111';
}
echo json_encode($arr);      // ["a111","b111","c111"]，这里改变了原来数组的值

$arr = ['a', 'b', 'c'];
foreach ($arr as $key => &$value) {
    $value = $value . '111';
}
 
foreach ($arr as $key => $value) {
    $value = $value . '222';
}
echo json_encode($arr);      // ["a111","b111","b111222222"]

$arr = ['a', 'b', 'c'];
foreach ($arr as $key => &$value) {
    $value = $value . '111';
}
unset($value);     // 释放$value的引用传递
foreach ($arr as $key => $value) {
    $value = $value . '222';
}
echo json_encode($arr);      // ["a111","b111","c111"]