You can do this by using GET variables which you will set in the url:
for single.php, url www.example.com.php?blog_no=121
if(isset($_GET["blog_no"])) {
// connect to database and get post
}
You need to set $_GET variable in the url when linking to that post: on the index.php page you would
<a href="single.php?blog_no=121">Post 121</a>