Well, I haven't worked very much with odbc (earlier experiences were too traumatic), but if the SQL is standard, your sql should look more like:
$sql = "INSERT INTO some_table (field_name1, field_name2, field_name3) VALUES ($sno, '$name', $marks)";
I couldn't tell from your sql what the name of the table was you are inserting the data into, or what the name of the fields are that you are putting the data into.
Also, not that I used single quotes around the string data because I am already using double qotes to delimit the string assignment ($sql="").
HTH
-- Rich Rijnders
-- Irvine, CA US