Select multiple options from option - php

Question

So I have a selector which gets its information from a database.
When I select something from the selector and press: Add to list, it generates a table with the selected information.
Now this what it should do. But now, when I select another result and press Add to list. It removes the old one and replaces it with the new one.
But I actually don't want it to remove the old one, but make a new row under it. So that table gets bigger. How do I do this?
Code for selector:
<!--Selector-->
<?php
//Get name and id data from the db. In an assoc array
$results = $database->Selector();
echo "<form name='form' method='POST' id='selector'>";
echo "<select name='train_name' id='train_name' multiple='multiple'>";
// Loop trough the results and make an option of every train_name
foreach($results as $res){
echo "<option value=" . $res['train_name'] . ">" . $res['train_name'] . "</option>";
}
echo "</select>";
echo "<br />" . "<td>" . "<input type='submit' name='Add' value='Add to list'/>" . "</td>";
echo "</form>";
if(isset($_POST["train_name"])){
//Get all data from database, in an assoc array
$results = $database->getAllAssoc();
//Make table headers
?>
<div id="train_select_table">
<form name="selector" method="post" action="customer_list.php?user_id=<?php echo $_SESSION['user_id']?>">
<table>
<tr>
<th>Train name</th>
<th>Number of bogies</th>
<th>Number of axles</th>
<th>Delete</th>
<th>More info</th>
<th>Check</th>
<!--Only for admins (later!)-->
<!--<th>XML</th>
<th>SQL</th> -->
</tr>
<div id="looprow">
<?php
foreach($results as $res){
//Loop trough results, generate a tablerow every time
?>
<tr>
<td name="train_name"><?php echo $res['train_name']?></td>
<td><?php echo $res['number_of_bogies']?></td>
<td><?php echo $res['number_of_axles']?></td>
<td>Delete</td>
<td>More Information</td>
<td><input type="checkbox" name="checkbox[]" value="<?php echo $res['train_id']?>"></td>
<!--Only for admins (later!)-->
<!--<td>XML</td>
<td>SQL</td>-->
</tr>
<?php
}
?>
</div>
</table><br />
<input name="Add to list" type="submit" id="add_to_list" value="add_to_list">
</form>
</div>
<?php
}
?>
Function:
function getAllAssoc() {
$sql = "SELECT * FROM train_information WHERE train_name = :train_name";
$sth = $this->pdo->prepare($sql);
$sth->bindParam(":train_name", $_POST["train_name"]);
$sth->execute();
return $sth->fetchAll();
}
function selector() {
$sql = "SELECT train_name, train_id FROM train_information";
$sth = $this->pdo->prepare($sql);
$sth->execute();
return $sth->fetchAll();
}
I understand why it keeps replacing the old row. this is because I send a new query. but I don't know how to keep the old one.

Answer 1

as each time user will chose a single value while not mutiple value, so if you want to use the new value and the old value too, you should store the old value using cookie or session or hidden input in that form. It depends on you.
E.g. at the beginning, do this:
<?php
session_start();//make it be before your output if you want to use session or cookie
if(!$_SESSION['train_name']) $_SESSION['train_name'] = array();
if(isset($_POST["train_name"])) $_SESSION['train_name'] = array_merge ($_SESSION['train_name'], $_POST["train_name"]);
.......
//next step, inquiry with $_SESSION['train_name']

Answer 2

Simply change the name of your multiple select list from train_name to train_name[] as follows:
...
echo "<select name='train_name[]' id='train_name' multiple='multiple'>";
...
By this way your $_POST['train_name'] will be an array passed to bindParam