<?php

if (isset($_POST['submit'])) {

    echo $_POST['account'];
}

?>

<form action="" method="POST">
    <select id="account" name="account">
        <option value="1">Tes1</option>
        <option value="2">Tes2</option>
    </select>
    <input type="submit" name="submit"/>
</form>

<?php
    if (isset($_REQUEST['save'])) {
        $sql = "SELECT Business_Unit, COUNT(Joining_Month) AS 'A' FROM jl WHERE Business_Unit = '".$_REQUEST['opt']."' GROUP BY Business_Unit";

        // Here is your sql query
        echo $sql;exit();
    }
?>

<form method="post" action="">
    <select name="opt">
        <option value="1">Option 1</option>
        <option value="2">Option 2</option>
        <option value="3">Option 3</option>
        <option value="4">Option 4</option>
    </select>

    <input type="submit" name="save" />
</form>