Codeigniter 3.1.8 JSON Post fails

Have you tried grabbing another source/website with file_get_contents() to confirm it's actually activated?
Have you tried making a POST without https? In case your Cross-Origin Resource Sharing (CORS) are set wrong. Don't know if this actually apply to Postman thought.
Do you have the latest Codeigniter?
Do you have the latest PHP version? (7.1 or 7.2)

My PHP version is .7.0.28
Codeigniter version is 3.1.8
The following code snippet works.

PHP Code:

<?
    var_dump(json_decode(trim(file_get_contents('php://input')), true));
?>

Posting {"user":1} gives me:
array(1) {
  ["user"]=>
  int(1)
}

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