$metin = '<img title="kelime1" src="http://yogaposes8.com/wp-content/uploads/2016/02/move-of-the-day-fish-pose-matsyasana-peaceful-dumpling.jpg" alt="kelime1" />';


preg_match_all('@alt="(.*?)"@si',$metin,$ayir);

$kelime = $ayir[1][0];

$t = array($kelime);
$t1 = array("<strong>$kelime</strong>");
$degistir = str_replace($t,$t1,$metin);

echo $degistir;



Bu şekilde istediğini yapabilirsin hocam