Fill Textbox if Checkbox Ticked?

[bjblackmore]

Hi,
How would i make sure a textbox was filled in, if a checkbox was ticked! But if it wasn't ticket, the textbox didn't have to be filled in? Or if a dropdown menu = a value, then a text box must be filled in. i.e. credit card menu = swtich, so isuue number must be filled in!
Cheers
Ben

[amcgrath]

if ($checkbox && !$textField) {

// do your thing

}