> I feel like I want to test what happens if you put values like "", false, " ", 0, an empty array, etc etc etc in front of ??.

All of those would result in the variable being assigned those values: the null check uses the same semantics as is_null(), and the truth table for those values is:

    $v     is_null($v)
    ------------------
    ''     FALSE
    false  FALSE
    ' '    FALSE
    0      FALSE
    []     FALSE

If you want to all falsy values to use the fallback value, use the shorthand ternary operator (?:) instead.

Only null will return the default value. http://3v4l.org/jphnq