PHP if/else

How do I get a radio button in 1 group to be checked when a radio button in another group becomes checked?
For example: I want the button in var1 w/value “No voice” to become automatically selected when button var2 w/value “$39.99” is selected.
<input type=“radio” name=“var1” value="$39.99 More" checked=“checked”>
<input type=“radio” name=“var1” value=“No voice”>
<input type=“radio” name=“var2” value="$29.99" checked=“checked”>
<input type=“radio” name=“var2” value="$39.99">

Is using if/else statements the right way? Would I also create more php if/else statements for other radio button scenarios?
Thanks for any help on this.