required field value

i have a form with validation toolkit.

one of the fields is set so that a value is not required, so the form is submitted if the field contents are either empty or in the required format.

what i would like to do is set the requirement based on the value in another field

for example, first field is option list, with option1 set as default.
if option 1 is selected, second field does not require a value.

if however option2 is set in my option list, second field does require a value.

the server side validation occurs within the if (isset($_POST["insert"])) {} block

is the best way to do this just to put the validate code within an if block ?

if (((isset($_POST["xxxxx"]))?$_POST["xxxxx"]:"") <> 0) {
$WAFV_Errors .= WAValidateNM(((isset($_POST["xxxxx"]))?$_POST["xxxxx"]:"") . "",0,1000000,2,",.",true,13);
} else {
$WAFV_Errors .= WAValidateNM(((isset($_POST["xxxxx"]))?$_POST["xxxxx"]:"") . "",0,1000000,2,",.",false,13);
}

This is done using client side validation, and using the Conditional Validation response.

Create the list with 2 options. Set the value for option 1 to be blank:
<option value="">option 1</option>

Then open the behaviors panel, click the plus button and apply the following validations:
Required / Not Blank on the select list
Conditional validation: Trigger = Select List. target = text Field
Required / Not Blank on the text field
Show Validation Errors

the key to understanding the different behaviors, is that everything In the Client validations menu like Alpha numeric or Credit card or required not blank is an action.

The behaviors in the main menu like show validation errors, conditional validation or enable disable form element are responses.

I am attaching a sample page to demonstrate.

thanks for the reply Jason.

I was going to look at client side validation after the server side, but you beat me to it.

one thing though, my existing fields all have spry client side validation, is your method compatible or do i have to take another appraoch

client side and spry validation can be applied to the same form fields, the forms created by form builder use the two at the same time along with server validation too.