How Do You Say "If This Array Contain This Value"?

I have the following in a server action:

$value - the id of an item e.g. 123
response - a comma separated list of ids, e.g. 120,123,125

I want to create a condition in a server action which says this:

If $value is one of the values in response

Does anyone know the code to do that?

Thanks!
Antony.