simple question abt convert to integer

my code is this: $query1="select * from patient where id=".$_POST['txt']; it works but i think because i have error in next line: *Warning*: mysql_num_rows() expects parameter 1 to be resource, boolean given $num2=Mysql_num_rows($result1); i echoed $ query1 and the result was this=>select * from patient where id=1 maybe it can't convert $_POST['txt'] from String to integer and then it can't get my $num it is int in my mysql how can i correct $query1?