Ключи массива могут быть либо инт либо стринг.
Все остальное приводится к ним.
В твоем случае приводится к инт = 0, первое значение перекрывается вторым.
Узнать шокирующие подробности:
php.net/manual/en/language.types.array.php
The key can either be an integer or a string. The value can be of any type.
Additionally the following key casts will occur:
Strings containing valid integers will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.
Floats are also cast to integers, which means that the fractional part will be truncated. E.g. the key 8.7 will actually be stored under 8.
Bools are cast to integers, too, i.e. the key true will actually be stored under 1 and the key false under 0.
Null will be cast to the empty string, i.e. the key null will actually be stored under "".
Arrays and objects can not be used as keys. Doing so will result in a warning: Illegal offset type.
