What are common issues when passing data to a PHP file using Ajax?

One common issue when passing data to a PHP file using Ajax is not properly encoding the data before sending it. To solve this issue, you can use the `JSON.stringify()` method to encode the data before sending it in the Ajax request.

$data = json_decode(file_get_contents("php://input"), true);
// Now $data contains the decoded JSON data sent from the client