<?php
$string = '
<li id="menu-item-98" class="navigation-heading menu-item menu-item-type-custom menu-item-object-custom menu-item-98">
<a href="#">Main</a>
</li>';
$string = preg_replace('~<li.*?>\s*(.*?)\s*</li>~', "<div class='navigation-heading'>\n<span>\n <span>$1</span>\n</span>\n</div>", $string);
print $string;
PD9waHAKCiRzdHJpbmcgPSAnCjxsaSBpZD0ibWVudS1pdGVtLTk4IiBjbGFzcz0ibmF2aWdhdGlvbi1oZWFkaW5nIG1lbnUtaXRlbSBtZW51LWl0ZW0tdHlwZS1jdXN0b20gbWVudS1pdGVtLW9iamVjdC1jdXN0b20gbWVudS1pdGVtLTk4Ij4KICAgIDxhIGhyZWY9IiMiPk1haW48L2E+CjwvbGk+JzsKCiRzdHJpbmcgPSBwcmVnX3JlcGxhY2UoJ348bGkuKj8+XHMqKC4qPylccyo8L2xpPn4nLCAiPGRpdiBjbGFzcz0nbmF2aWdhdGlvbi1oZWFkaW5nJz5cbjxzcGFuPlxuICAgIDxzcGFuPiQxPC9zcGFuPlxuPC9zcGFuPlxuPC9kaXY+IiwgJHN0cmluZyk7CgpwcmludCAkc3RyaW5nOw==