Continuie button is missing in Oscommerce

Add the below code BEFORE "if (isset($navigation->path[$back])) {" in the shopping_cart.php
// BOF FWR Mod category based continue button
        $count = count($products);
        if( isset($products[$count-1]['id']) ) {
          $continueButtonId = tep_get_product_path(str_replace(strstr($products[$count-1]['id'], '{'), '', $products[$count-1]['id']));
        }
        if( isset($continueButtonId) ) {
?>
                                <td align="center" class="main"><?php echo '<a href="' . tep_href_link(FILENAME_DEFAULT, 'cPath=' . $continueButtonId) . '">' . tep_image_button('button_continue_shopping.gif', IMAGE_BUTTON_CONTINUE_SHOPPING) . '</a>'; ?></td>
<?php
    //if (isset($navigation->path[$back])) {
        } else

How do you disable warnings in PHP?

display_errors = Off
...in your php.ini file. i highly recommend that you don't do this on your development environment as this will nearly impossible to debug your code.

You can also use:
error_reporting(0);  at the top of your script.