How to get a query variable?

You should probably provide different default if empty string is indistinguishable from your value.

$myvar = get_query_var( 'myvar1', false );

if ( false !== $myvar ) {

    // stuff
}

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)