How to put phrases in strong

A filter should do the job, but it may cause performance problems. The code below is untested, but should be close. Add this to your functions.php:

<?php
function mam_strong_phrases ($content) {
   $phrase_patterns = array(
      '/(first phrase)/',
      '/(second phrase)/',
      '/(and a third phrase)/'
      );
   $result = preg_replace($phrase_patterns, '<strong>$1</strong>',$content);
   return $result;
}
add_filter('the_content','mam_strong_phrases');
?>