MySQL database could not be updated with PHP program - php

Question

addmember.php
<?php
require_once("dbtools.inc.php");
$account = $_POST["account"];
$password = $_POST["password"];
$name = $_POST["name"];
$sex = $_POST["sex"];
$year = $_POST["year"];
$month = $_POST["month"];
$day = $_POST["day"];
$telephone = $_POST["telephone"];
$address = $_POST["address"];
$email = $_POST["email"];
$comment = $_POST["comment"];
$link = create_connection();
$sql = "SELECT * FROM users Where account = '$account'";
$result = execute_sql($link, "member", $sql);
if (mysqli_num_rows($result) != 0)
{
mysqli_free_result($result);
echo "<script type='text/javascript'>";
echo "alert('Account already in use! Please choose another username');";
echo "history.back();";
echo "</script>";
}
else
{
mysqli_free_result($result);
$sql = "INSERT INTO users (account, password, name, sex,
year, month, day, telephone, address,
email, comment) VALUES ('$account', '$password',
'$name', '$sex', $year, $month, $day, '$telephone',
'$address', '$email', '$comment')";
$result = execute_sql($link, "member", $sql);
echo "User added successfully!";
}
mysqli_close($link);
?>
join.html
<form action="addmember.php" method="POST" name="myForm">
(Different types of input)
<input type="submit" value="Add">
My aim is to add a member data into the database after the user clicked the Add button on the form in join.html. However the page could run echo "User added successfully!"; this line but the problem is the database could not get updated even though I already called execute_sql command. May I ask what is missing in order to be connected with the database?