Why is the class named UintArray, while the constants are named like UInt8? Even if PHP is case-insensitive, it would be nice for you to be consistent in capitalization.

UIntDefault = 0 seems weird to me. Why not define UIntDefault = UInt32, since that's what you mean? Why is the bitmask for UInt32 0xFFFFFFFE instead of 0xFFFFFFFF? That rounds your data to even numbers.

When defining $bit_masks, there is no obvious correspondence with the UIntn constants that were just defined. I suggest:

private static $bit_masks = array(
    UInt8  => 0xFF,
    UInt16 => 0xFFFF,
    …
);

Why do offsets need to be sanitize()d? Do you not trust the caller to give you offsets that are integers?

The way you keep track of $current_count is buggy. In a normal array, you should be able to do $a = array(); $a[0] = "first"; $a[] = "second";. However, if you try that with your UintArray, the second entry will overwrite the first, because you only maintain a correct $current_count if you stick to the push()/pop()/offsetUnset() interface.

__sleep() is supposed to serialize the array, but you only serialize the data, and discard the type.

Why is the class named UintArray, while the constants are named like UInt8? Even if PHP is case-insensitive, it would be nice for you to be consistent in capitalization.

UIntDefault = 0 seems weird to me. Why not define UIntDefault = UInt32, since that's what you mean? Why is the bitmask for UInt32 0xFFFFFFFE instead of 0xFFFFFFFF? That rounds your data to even numbers.

When defining $bit_masks, there is no obvious correspondence with the UIntn constants that were just defined. I suggest:

private static $bit_masks = array(
    UInt8  => 0xFF,
    UInt16 => 0xFFFF,
    …
);

Why do offsets need to be sanitize()d? You want to validate the offset, not sanitize it. I don't recommend trying to make sense of bad input, because garbage in = garbage out. Some languages interpret negative array indices as counting backwards from the end — maybe you want to do that? But it's not idiomatic for PHP, so I recommend either emulating the standard behaviour for PHP arrays or explicitly failing.

The way you keep track of $current_count is buggy. In a normal array, you should be able to do $a = array(); $a[0] = "first"; $a[] = "second";. However, if you try that with your UintArray, the second entry will overwrite the first, because you only maintain a correct $current_count if you stick to the push()/pop()/offsetUnset() interface.

__sleep() is supposed to serialize the array, but you only serialize the data, and discard the type.