``` $arr=[[1,2], [3,4], [5,6]]; foreach ($arr as $key => &$value){ $value[0]="a"; } print_r($arr); ``` 结果: ``` Array ( [0] => Array ( [0] => a [1] => 2 ) [1] => Array ( [0] => a [1] => 4 ) [2] => Array ( [0] => a [1] => 6 ) ) ```