$kod = '<img src="http://www.siteadresi.com/wp-content/uploads/2017/09/waffle-tarifi.jpg" alt="Waffle" width="310" height="200" />';

 preg_match_all('@"(.*?).jpg"@si',$kod,$kod1); 
 
 $degistir = ''.$kod1[1][0].'-310x175.jpg';

echo $degistir;