I think there may bee something wrong with your sql-statement:
$mysqlcon = "select * from = '$start' reg_info where username = '$username';"
1) 'select from' chooses all data in the table specified.
2) you shouldn't write 'select from = , but 'select * from table_name'
3) what is the name of your table?
4) do you really want to choose all data in the rows that match your condition('where username='$username')??
I cannot understand where the variable $start comes from and why you put it in your sql-statement.
For your purpose, I guess i would have written one of these to sqls, depending on wether or not I needed all data fields from rows matching my condition(seems like you only need the 'username'-field = the last sql):
$sql = "SELECT * FROM reg_info WHERE username='$username'";
Now, to choose only the field 'username':
$sql = "SELECT username FROM reg_info WHERE username='$username'";