I've seen someone handle undefined variables and indexes with a custom getIfSet() function. If I recall correctly, it took a variable (e.g. $test, $test['key']), and an optional default value. Within the function it did a isset(), and if it was set, would return it's value, otherwise returning the null or the optional default value.
It would have looked something like this...
function getIfSet (&$var, $defaultValue = null)
I'm not saying that's the best solution, but it's A solution. Generally speaking though, global helper functions in an OO application are not recommended as objects become dependant on them, and as a result less portable. It's important however not to forget the reason this notice exists. It exists to assist in debugging, by alerting you of undefined variables which may be causing unpredictable behaviour, so only use checks such as isset() and getIfSet() if it's expected that the variable may not be defined. Using such checks anywhere else creates messy convoluted code, and defeats the point of having this notice.