<form method="post" action="<?php bloginfo('url'); ?>/customsearch.php" class="search-form" role="search"> ... </form>
customsearch.php in content :
customsearch.php should be here: yoursite.com/dir/customsearch.php
<form method="post" action="<?php bloginfo('url'); ?>/customsearch.php" class="search-form" role="search"> ... </form>
customsearch.php in content :
customsearch.php should be here: yoursite.com/dir/customsearch.php