Long time to evaluate a simple expression

If I use an expression similar to this:

 if ($Age>=21 and $Age<=31) echo "OK";

and set the value of $Age so that it lies within the specified range (e.g. $Age="23") the expression evaluates instantly.

If however, I set the value of $Age outside of the specified range, PHP takes about 40 seconds to do the evaluation?

Any ideas

I have found an answer to this peculiarity.

When I was running the code previously, all that I had on the page was the code itself i.e.

<?php

code....

If however, I add the necessary tags to make an HTML page:

<?php

code...

</body>
</html>

the expression evaluates instantly, whatever the value of $Age

In both cases the page was saved with a .php extension.

It would be interesting to know why this is so.