If someone tries to access this page:
www.myhomepage.com/m/40921
I want the URL (above) to remain the same but the content should be from this page:
www.myhomepage.com/msg?t=40921
If it must be exactly like you said, then use this
RewriteRule ^m/([0-9]+)\/?$ msg?t=$1
But I think you made a typing error. Shouldn't it be msg.php? If so, use this:
RewriteRule ^m/([0-9]+)\/?$ msg.php?t=$1
And in your php file, you can get the t with $_GET['t'].