in the original code, you see the usage of thi is_empty function:
function isempty( $postvar , $title , $required = 0 , $min=0 , $max=0)
The first parameter is the post var name ,
second is the title, that will be in the error message.
3th parameter is 1 if that field is required , 0 if its not.
4th you set the minimum lenght of that field,
5th sets the maximum lenght of that field.
if you set the lenghts into 0, the function won't check the lenghts!
isempty( "ask", "ask", 1 , 0,0);
isempty( "Polypipo", "Polypipo", 1 , 0,0);
isempty( "surname", "surname", 0 , 0,0);
...
...
is_mail("email");
is_mail: this created function checks a post variable using the parameter.
and fills the error_message string.