Code Question About For Loops

I am Trying to write a for loop and allow the use of adding more to the block why is this erroring?

<div id="<?php block_field('custom-id'); ?>" class="btn-group <?php block_field('custom-class'); ?>" role="group">
    <?php
    $BTN_NUM = block_value('number-buttons');
    for ($i = 0; $i < $BTN_NUM; $i++) {
        echo '<button href="' + block_field('button-url-' + $i ) + '" type="button" class="btn ' + block_field('button-type-' + $i ) + '">' ;
            block_field('button-text-' + $i );
        echo '</button>' ;
    }
    ?>
</div>