JS & PHP - Change dropdown value if other fields are filled - php

Question

I have a PHP application form for medical health plans where the user fills in his details and selects some options from various controls, one of which is the plan type (private of family plan). Sometimes, although the user fills in his siblings/spouse details, they forget to also select the 'family' option and leave the drop down at it's default value which is the 'private' plan.
How can I make sure that if the user fills in his sibling's details, that the 'plan' dropdown will change to the proper option? I know this can be done easily with js but I'm not sure what event I should call it on, onFocus and onClick won't work because the user might simply click on the Sibling Name field but not enter a value, fill in something but later on clear it out etc.
I think that a script that counts the length of the 4 available siblings fields and if one of them is not empty, then change the 'plan' select value from 'private' to 'family'.
Any ideas would be great, thanks.

Answer 1

You've givin the answer partially yourself: you'll need a function that checks if the fields are not empty (or some other random condition). Lets call that function checkCondition()
Next you'll need a validation function that calls on it and changes the fields if appropriate, you can also combine these two functions but the code usually looks cleaner if you seperate them. E.g. ensureValidFieldPlan()
Then you can hook that function to run on the appropriate times. The simplest way to do it is in the forms onChange. A better way to do it would be to hook it to the appropriate form field, but for simplicity sake lets hook it to the form.
Your total code will look something like this:
function checkCondition(){
return form.field1.value.length != 0;
}
function ensureValidFieldPlan(){
if( form.plan.value != 'family' && checkCondition()){
form.plan.value = 'family'
}
}
form.onchange = function(){ form.onChange(); ensureValidFieldPlan(); };
Note that this does not guarantee that a browser client will transfer a valid form (e.g. it can be spoofed), you should always also perform server-side validation.

Answer 2

Depends on how you want to handle it, server side? Indeed, detect the
strlen($_POST['siblingname']) > 0
and then set your plan automatically to familly from the php code before processing something else.
If you want to do it on the client side. I suggest to use the popular JQuery library and intercept the value change event on the sibblingname field. Then, in the event handler, check the length of the sibbling name field value, if there is something, just set the plan to family.
Good luck