PHP数组到字符串转换对象方法的访问

Question

$config = [..., ..., ..., 'myMethod'];

$this->$config[3]($models); //this works on local but (Array to string conversion on server)

$method = $config[3];
$this->$method($models); // Now works on server

在php 5.6上的工作在php 7上失败

出什么问题了？