How to skip empty field values ​​when submitting a form in PHP?

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I have a form that echoes values for a specific user from a MySQL database using PHP. I am trying to figure out how to allow the user to submit the form to update their user information but have the form skip any field they have no filled out.

Current update statement

if (!isset($_POST['btnLogin'])) {
    $db = DB();
    $stmt = "UPDATE users SET fName = :fName,
                              lName = :lName,
                              emailAddress = :emailAddress
                              WHERE user_id = $user->user_id";
    $query = $db->prepare($stmt);
    $query->bindParam(':fName', $_POST['fName'], PDO::PARAM_STR);
    $query->bindParam(':lName', $_POST['lName'], PDO::PARAM_STR);
    $query->bindParam(':emailAddress', $_POST['emailAddress'], PDO::PARAM_STR);
    $query->execute();
};

Form echoing user information

<form class="form-horizontal" action="profile.php" method="post">
                <div class="form-group">
                    <label class="col-lg-3 control-label">First name:</label>
                    <div class="col-lg-8">
                        <input class="form-control" type="text" name="fName" placeholder="<?php echo $user->fName ?>"/>
                    </div>
                </div>
                <div class="form-group">
                    <label class="col-lg-3 control-label">Last name:</label>
                    <div class="col-lg-8">
                        <input class="form-control" type="text" name="lName" placeholder="<?php echo $user->lName ?>">
                    </div>
                </div>
                <div class="form-group">
                    <label class="col-lg-3 control-label">Email:</label>
                    <div class="col-lg-8">
                        <input class="form-control" type="email" name="emailAddress" placeholder="<?php echo $user->emailAddress?>">
                    </div>
                </div>
                <div class="form-group">
                    <label class="col-md-3 control-label">Username:</label>
                    <div class="col-md-8" style="margin-top: 7px;">
                        <?php echo $user->username ?>
                    </div>
                </div>
                <div class="form-group">
                    <label class="col-md-3 control-label"></label>
                    <div class="col-md-8">
                        <input class="btn btn-primary" name="btnUpdate" value="Save Changes" type="button">
                        <span></span>
                        <input class="btn btn-default" value="Cancel" type="reset">
                    </div>
                </div>
            </form>

Currently this doesnt appear to update the database at all. If I leave the form completely blank and submit it, the values that existed in the database are now empty. i.e., just blank columns.

I have been looking at other examples on how to do this but I cannot seem to figure this out. Any help would be appreciated.

Just to be sure I am in fact updating the correct user, I made sure that my $user->user_id statement is in fact returning the correct user_id from the database for the update.

UPDATE

Currently this is how I have the update statement / code

if(!empty(['btnUpdate'])) {
$stmt = "UPDATE users SET fName = IF(:fName = '', fName, :fName),
                lName = IF(:lName = '', lName, :lName),
                emailAddress = IF(:emailAddress = '', emailAddress, :emailAddress)
        WHERE user_id = $user->user_id";
$db = DB();
$query = $db->prepare($stmt);
$query->bindParam("fName", $fName, PDO::PARAM_STR);
$query->bindParam("lName", $lName, PDO::PARAM_STR);
$query->bindParam("emailAddress", $emailAddress, PDO::PARAM_STR);
$query->execute();
}

Using <?php var_dump($_POST) ?> returns 0 after submitting the form and I still end up with empty database columns


This isn't ignoring empty fields, but a more user friendly solution would be to stick the database values into each input's value attribute. That way, you don't have to skip the fields if they're blank, and a user won't get confused when editing their data. If you don't want them to be able to edit the value, just include the readonly attribute.

If you don't want to do this, than you could either create a dynamic query or use an if statement to decide which value to use, the form's or the database's.