Both Java and JavaScript use -1 when an index isn't found. Since the index is always 0-n it seems a pretty obvious choice.
//JavaScript
var url = 'example.com/foo?bar&admin=true';
if(url.indexOf('&admin') != -1){
alert('we likely have an insecure app!');
}
I find this approach (which I've used when extending Array-type elements to have a .indexOf() method) to be quite normal.
On the other hand, you can try the PHP approach e.g. strpos() but IMHO it gets confusing as there are multiple return types (it returns FALSE when not found)
