Code will trim a string to a maximum length, and will not break sentences (will grab as many sentences from text as it can up to the maximum string length).

gistfile1.php

<?php

    function trim_text_in_sentences($intLength, $strText) {
       $intLastPeriodPos = strpos(strrev(substr($strText, 0, $intLength)), '.');
       if ($intLastPeriodPos === false) {
           $strReturn = substr($strText, 0, $intLength);
       } else {
           $strReturn = substr($strText, 0, ($intLength - $intLastPeriodPos));
       }
       return $strReturn;
   }

?>