$file_name = "";
preg_match_all("/<img[^>]*src=[\"']?([^>\"']+)[\"']?[^>]*>/i",$content,$url_mov);

for($i=0;$i<count($url_mov[1]);$i++){
     $file_name_ext = substr($url_mov[1][$i],strrpos($url_mov[1][$i],"."));//확장자이름
     $file_name_ext = trim(str_replace(".","",$file_name_ext));
     $file_name = $url_mov[1][0].$url_mov[1][1]; //파일이름
     if($file_name_ext){
           $content = preg_replace("/<img[^>]*src=[\"']?([^>\"']+)[\"']?[^>]*>/i","변환할 내용",$content);
      }
}

echo htmlspecialchars($content);