<?php
$str = 'Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. ...';
$start = 'sadipscing';
$end = 'dolore';
$pattern = sprintf(
'/%s(.+?)%s/ims',
preg_quote($start, '/'), preg_quote($end, '/')
);
if (preg_match($pattern, $str, $matches)) {
list(, $match) = $matches;
echo $match;
}