Problem with HTTP POST with SIM808

Hello,

I'm trying to post data to a MySQL database with the following AT commands:
AT+SAPBR=3,1,"Contype","GPRS"
AT+SAPBR=1,1
AT+HTTPINIT
AT+HTTPPARA="CID",1
AT+HTTPPARA="CONTENT","application/x-www-form-urlencoded"
AT+HTTPPARA="URL", "http://webaddress.com/data2.php"
AT+HTTPDATA=23,15000

device_id=100001

AT+HTTPACTION=1
AT+HTTPREAD=0,400
AT+SAPBR=0,1
AT+HTTPTERM

The response from AT+HTTPACTION=1 is +HTTPACTION: 1,200,0
There is no error but the data is not written to the database table. If I submit the data through a form in a browser the data is written to the table so I believe the PHP file is working fine. The PHP file is below:

<?php include 'connect.php'; $query = "INSERT INTO test2 (device_id) VALUES (?)"; $stmt = mysqli_prepare($dbc, $query); mysqli_stmt_bind_param($stmt, "s", $_POST["device_id"]); mysqli_stmt_execute($stmt); mysqli_close($dbc); ?>

Maybe the AT+HTTPPARA="CONTENT","application/x-www-form-urlencoded" is incorrect?
Or the format of the data provided is in correct: device_id=100001

Thank you in advance for any advice.

The problem is with the bind statement in the php file. The data type must be set to "i".