file_get_contents（“php：// input”）返回String（0）

Im receiving JSON data using PHP.
I'm using file_get_contents("php://input") but it doesn't work.
It returns String(0);.
This is my code:

echo $content = file_get_contents("php://input");
//Attempt to decode the incoming RAW post data from JSON.
$decodedarray = json_decode($content, true);

I have looked into my php.ini file and allow_url_fopen returns 1.
I don't know what I'm doing wrong.