我如何找出mysqli扩展有什么问题？

I'm using PHP 5.6.30 and I need mysqli extension. In php.ini I have

extension_dir = "ext"
...
extension=php_mysqli.dll

ext dir exists in folder where php is installed, and php_mysqli.dll exists in either ext folder and in php install folder. php -m shows that mysqli is installed. Still the following code

new mysqli ( 'localhost', 'root', 'root', 'db_name_here');

outputs Fatal error: Class 'mysqli' not found

The question is: how can I find out what's wrong with mysqli

UPD. I'm using Win7 and the server is Apache 2.4

You can also check if the library is loaded or not in your env

  if (extension_loaded('mysqli') && function_exists('mysqli_init')) {
   echo "having lib";
  }else{
  echo "No library loaded or exist";
  }

Also you can specify fully qualified path for extension_dir.

Check your extension_dir path. It should be fully qualified. i.e

   extension_dir = "C:/wamp/php/ext" // your php path folder.