void postData()
{
  sim900a.println("AT+HTTPINIT");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPPARA="CID",1");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPPARA="URL","test.my/demo/writedata.php"");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPDATA=12,10000");
  delay(1000);
  printSerialData();

  sim900a.println("strBoton=999");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPACTION=1");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPREAD");
  delay(1000);
  printSerialData();

  sim900a.println("AT+HTTPTERM");
  delay(1000);
  printSerialData();
}

<?php
$servername = "test.my";
$username = "****";
$password = "****";
$dbname = "test";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$Boton = $_GET["strBoton"];

$sql = "INSERT INTO test VALUES ('$Boton')";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}
$conn->close();
?>

AT+HTTPINIT
OK

AT+HTTPPARA="CID",1
OK

AT+HTTPPARA="URL","test.my/demo/writedata.php"
OK

AT+HTTPDATA=12,10000
DOWNLOAD
OK

AT+HTTPACTION=1
OK

AT+HTTPREAD
OK

AT+HTTPTERM
+CME ERROR: operation not allowed

AT+SAPBR=0,1
OK