Boostrap 4 Form Validation

Bootstrap 4 From Validation

<?php
$field_name = 'field';
$field_label = 'Field';
?>

//INPUT FIELD 

<div class="form-group <?php echo $_ERROR[$field_name] ? ' has-danger' : ''; ?>">
    <label for="<?php echo $field_name; ?>"
           class="col-sm-0 control-label"><?php echo $field_label; ?></label>
    <div class="col-sm-0">
        <input name="<?php echo $field_name; ?>" type="text" class="form-control"
               id="<?php echo $field_name; ?>"
               value="" placeholder="">
        <?php echo $_ERROR[$field_name] ? '<span class="help-block">' . $_ERROR[$field_name] . '</span>' : ''; ?>
    </div>
</div>

//TEXTAREA FIELD

<div class="form-group <?php echo $_ERROR[$field_name] ? ' has-danger' : ''; ?>">
    <label for="<?php echo $field_name; ?>"><?php echo $field_label; ?></label>
    <div class="col-sm-0">
        <textarea  name="<?php echo $field_name; ?>" class="form-control" id="<?php echo $field_name; ?>" cols="100" rows="3"></textarea>
        <?php echo $_ERROR[$field_name] ? '<span class="help-block">' . $_ERROR[$field_name] . '</span>' : ''; ?>
    </div>
</div>

//SELECT FIELD

<div
    class="form-group <?php echo $_ERROR[$field_name] ? ' has-danger' : ''; ?>">
    <label class="control-label"
           for="method"><?php echo $field_label; ?></label>
    <select class="form-control" name="<?php echo $field_name; ?>" id="<?php echo $field_name; ?>">
        <option value="stripe">Credit / Debit Card</option>
    </select>
</div>

<?php

if ($_SERVER['REQUEST_METHOD'] == 'POST') {

    $_ERROR = array();

    if (strlen($_POST['field']) == 0){
        $_ERROR['field'] = 'Message';
    }
    
    if (!count($_ERROR)) {

        $field = $_POST['field'];
        
    } else {
        $_SESSION['message']['error'] = 'Please correct the highlighted fields and submit again';
    }
}

?>