<?php
 
$string ='Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam vitae sem sem. Praesent quis fringilla massa. Nullam in arcu id eros semper imperdiet. Pellentesque consectetur facilisis quam, dictum euismod sem elementum a. Nam lacinia velit vel lectus rhoncus, in vulputate augue lacinia. Nullam nec dignissim velit, sit amet porta diam. Nullam leo enim, dictum at ante eu, suscipit sollicitudin elit. Sed ligula felis, molestie sed commodo commodo, dapibus a nibh. Etiam tellus dui, hendrerit nec purus in, rutrum convallis odio. Donec bibendum sapien justo, ut bibendum risus lobortis at. Quisque ipsum justo, molestie sed cursus eu, sodales ac quam. In urna leo, faucibus at luctus ut, euismod viverra metus. In eu nibh eget magna pulvinar ullamcorper in sed mi.';
$string = chunk_split($string, 80, PHP_EOL);
 
echo strlen($string) , ': ', $string, PHP_EOL, PHP_EOL;
 
$output = preg_replace('/(^.{0,249}[^!?.]*.).*$/s', '$1', $string);
 
echo strlen($output) , ': ', $output, PHP_EOL, PHP_EOL;

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