I think I understand your problem.

You send in the form several fields:

<input type = 'hidden' name = 'copySession' ...>

which have the same name and not in table form! so it is normal that when receiving the request:

$_POST["copySession"]

you get only one and therefore the first.

If you want to send them all, you have to do:

<input type = 'hidden' name = 'copySession[]' ...>

and you get the request as an array.

foreach($_POST["copySession"] as $sessionId){ ... }

If you want to send only one field, you must make them disabled with javascript in real time during the selection.

For example you put in all fields copySession disabled and you add a class to them. Then you add the same class on the button as well and when the user clicks on a button, the field concerned removes the disabled.

With jQuery something like:

//Click on button
$('a.specialClass').on('click', function(){

    //Disabled all copySession inputs
    $('input[name="copySession"]').prop('disabled', true);

    //Let the field concerned able to be send 
    $('input.specialClass[name="copySession"]').prop('disabled', false);
});

Good luck!

I’m redoing the whole post since I had hard time explaining the issue or question.

What this code does:
The user can create a new training session. They can name it and if they want to, they can copy the content from previously created session.

I’m using Bootstrap 4 list group items to show the previous sessions. My problem is that I can not catch the user selection to post the data to activateSaveTrainingSession.php, which includes the SQL query to insert the new data to the database.

I can pass the data from the form to the action php file from inputSessionName -input. As you can see, I’ve also tried using input type=”hidden”. It kind of works, but it only uses the $sessionId from the first row it fetches, not the user selection. And thats the problem: how do I catch which list item the user selects, so I can post the data to the activateSaveTrainingSession.php?

<div class="row">
    <div class="col-lg-12">
        <form class="was-validated" action="activateSaveTrainingSession.php" method="post">
            <div class="custom-control">
                <div class="form-group">
                    <label for="inputSessionName" class="float-left"><?php echo $lang['TRAINING_SESSIONNAME_HEADER'] ?></label>
                    <input type="text" class="form-control" id="inputSessionName" name="inputSessionName" placeholder="Example 1" minlength="3" maxLength="128" required>
                    <small id="inputSessionNameHelp" class="form-text text-muted">
                        <?php echo $lang['TRAINING_SESSIONNAME_HELPTEXT'] ?>
                    </small>
                </div>
            </div>
            <h5 class = "mt-3"><?php echo $lang['TRAINING_COPYSESSION_HEADER'] ?></h5>
            <div class="row mt-3">
                <div class="col-lg-6 mb-3">
                    <div class="list-group" id="list-tab" role="tablist">
                        <a class="list-group-item list-group-item-action active" id="list-doNotCopy-list" data-toggle="list" href="#list-doNotCopy" role="tab" aria-controls="list-doNotCopy">Do not copy</a>
                        <?php
                        $stmt = $link->prepare('SELECT `id`, `sessionName`, `createDate` FROM `trainingSessions` WHERE `userId` = ? ORDER BY `id` DESC LIMIT 5');
                        $stmt->bind_param('i', $currentUserId);
                        $stmt->execute();
                        $stmt->store_result();
                        $stmt->bind_result($sessionId, $sessionName, $sessionNameDate);
                        $sessionCounter = 0;
                        $tabsArray = array();
                        if ($stmt->num_rows > 0) {
                            while ($stmt->fetch()) {
                                $sessionNameDateFixed = date("d.m.Y", strtotime($sessionNameDate));
                                $sessionCounter += 1;
                                $listId = "list-$sessionId-list";
                                $tabId = "list-$sessionId";
                                array_push($tabsArray, $sessionId)
                                ?>
                                <a class="list-group-item list-group-item-action" id="<?php echo $sessionId ?>" data-toggle="list" href="#<?php echo $tabId ?>" role="tab" aria-controls="<?php echo $tabId ?>"><?php echo $sessionName ?><input type='hidden' name='copySession' value='<?php echo $sessionId ?> '/></a>
                                <?php
                            }
                        } else {
                            echo "No results.";
                        }
                        $stmt->close();
                        echo "Displaying last $sessionCounter records.";
                        ?>
                    </div>
                </div>
                <div class="col-lg-6">
                    <div class="tab-content" id="nav-tabContent">
                        <div class="tab-pane fade" id="list-doNotCopy" role="tabpanel" aria-labelledby="list-doNotCopy-list">Do not copy data from previous session.</div>
                        <?php
                        foreach ($tabsArray as $session) {
                            $tabsTextArray = array();
                            $stmt = $link->prepare('SELECT trainingSessions.id, workouts.workoutName, exercises.setNumber, exercises.reps, exercises.weights FROM workouts INNER JOIN exercises ON workouts.id = exercises.workoutId INNER JOIN trainingSessions on trainingSessions.id = exercises.sessionId WHERE exercises.userId = ? AND trainingSessions.id = ? ORDER BY exercises.id DESC');
                            $stmt->bind_param('ii', $currentUserId, $session);
                            $stmt->execute();
                            $stmt->store_result();
                            $stmt->bind_result($sessionId, $workoutName, $set, $reps, $weights);
                            if ($stmt->num_rows > 0) {
                                while ($stmt->fetch()) {
                                    $tabText = "<strong>$workoutName</strong> sarja $set, $reps x $weights kg<br>";
                                    array_push($tabsTextArray, $tabText);
                                }
                            }
                            $listId = "list-$sessionId-list";
                            $tabId = "list-$sessionId";
                            ?>
                            <div class = "tab-pane fade" id="<?php echo $tabId ?>" role = "tabpanel" aria-labelledby = "<?php echo $listId ?>"><?php
                                foreach ($tabsTextArray as $text) {
                                    echo "$text";
                                }
                                ?>
                            </div>
                            <?php
                        }
                        $stmt->close();
                        ?>
                    </div>
                </div>
            </div>
            <button type="submit" name="buttonSaveTrainingSession" class="btn btn-success float-left"> 
                <i class="fas fa-sd-card"></i>
                <?php echo $lang['TRAINING_BTN_SAVE'] ?>
            </button>
        </form>
    </div>
</div>

activateSaveTrainingSession.php

<?php

require_once('config/sql.php');
include_once('config/common.php');
if (!isset($_SESSION["loggedin"]) || $_SESSION["loggedin"] !== true) {
    header("location: login.php");
    exit;
}

$currentUserId = $_SESSION["currentUserId"];

$submitButton = strip_tags(trim($_POST['buttonSaveTrainingSession']));
if (isset($submitButton)) {
    $inputSessioName = filter_var(trim($_POST["inputSessionName"]), FILTER_SANITIZE_FULL_SPECIAL_CHARS);
    $copySessionID = filter_var(trim($_POST["copySession"]), FILTER_SANITIZE_NUMBER_INT);

    if (empty($inputSessioName) && strlen($inputSessioName) < 3 && $inputSessioName > 128) {
        header("location: training.php?msg=invalidSessionName");
    } else {
        echo "Name: $inputSessioName <br>";
        echo "ID: $copySessionID";
    }
} else {
    header("location: 404.php");
}
?>