$yerine_koyulacak = 'hede.jpg';
$cikti = preg_replace('#thumbUrl=(.*?)&amp;#', 'thumbUrl=' . $yerine_koyulacak . '&amp;', '#embed height="322" width="512" src="http://d.yimg.com/static.video.yahoo.com/yep/YV_YEP.swf?ver=2.2.46" type="application/x-shockwave-flash" flashvars="id=15914254&amp;vid=&amp;lang=en-us&amp;intl=us&amp;thumbUrl=http%3A//l.yimg.com/a/p/i/bcst/videosearch/11679/94587168.jpeg&amp;embed=1" bgcolor="#000000" allowscriptaccess="always" allowfullscreen="true"></embed></div>#');
echo $cikti;