仅针对一个索引：

$arrayTwoD = array();
foreach ($valueMult[0]['User'] as $key => $value) { 
    $arrayTwoD[$key] = array_push(explode(',', $value));
}

如果$multArray中有多个索引：

$arrayTwoD = array();
foreach ($multArray as $keyMult => $valueMult) { 
    foreach ($valueMult['User'] as $key => $value) { 
        $arrayTwoD[$keyMult][$key] = array_push(explode(',', $value));
    }
}

或

$arrayTwoD = array();
foreach ($multArray as $array) { 
    foreach ($array['User'] as $key => $value) { 
        $arrayTwoD[$key] = array_push(explode(',', $value));
    }
}

试试这个

$array = array(...); // your array data
$formedArray = array();

foreach ( $array as $arr )
{
    foreach ( $arr['user'] as $key => $value )
    {
        $formedArray[$key] = array_push(explode(",",$value));
    }
}

echo "<pre>";
print_r($formedArray);
echo "</pre>";

我知道这有点重复，但你也可以这样做：

foreach($array as $users) {
    foreach($users as &$value) { // &value is assigned by reference
        $users['users']["email"] = explode(",", $value['email']);
        $users['users']["username"] = explode(",", $value['username']);
        $users['users']["description"] = explode(",", $value['description']);
    }
}

但是在那之后，你需要使用$value，参考官方的PHP manual documentation来了解更多关于&符号在这里的作用。
Demo

使用array_map()可以用来访问子集数据（无需在全局作用域中声明任何新变量），并对preg_split()进行迭代调用以将分隔值分隔为子数组。
代码：（Demo）

var_export(
    array_map(
        fn($csv) => preg_split('/ ?,/', $csv),
        $array[0]['users']
    )
);