I know this comes late, but I wrote in my comments earlier that mysql is deprecated and it is not good to used it and most answer is based on deprecated method.
The risk of using mysql is that when you filter the jobs you can risk some body inject sql statement and return all job list, and that might overload your server in this case.
The other thing is your variable names can get you confused in long term, I will use understandable names.
Here is how your code should look like:
<?php
$db_hostname = 'localhost';
$db_database = 'site';
$db_username = 'root';
$db_password = '';
$db_table = 'tablesite';
$job = '';
// Create connection
$conn = new mysqli($db_hostname, $db_username, $db_password, $db_database);
// Check connection
if ($conn->connect_error)
die("خطا در اتصال به پايگاه داده" . $conn->connect_error);
$job = mysqli_real_escape_string($conn, $job);
//since user and password is not blank, find user info using the email and password entered by user
$sql = "SELECT * FROM `$db_table` WHERE job <> ?";
$stmt = $conn->prepare($sql);
$stmt->bind_param("s", $job);
$stmt->execute();
$output = $stmt->get_result();
echo "<select>";
while ($row = $output->fetch_array(MYSQLI_NUM))
{
echo "<option value=" . $row[0] . ">" . $row[1] . "</option>";
}
echo "</select>";