What is PHP's SPL data structures vs native arrays?

PHP's native arrays are incredibly versatile but are not always the most efficient data structure. SPL data structures offer typed, memory-efficient alternatives. SplFixedArray uses significantly less memory than a native PHP array for large integer-indexed arrays because it allocates a contiguous block of memory (like C arrays). SplStack and SplQueue are proper LIFO/FIFO implementations. SplMinHeap and SplMaxHeap provide O(log n) priority queue operations. SplDoublyLinkedList allows O(1) head/tail operations. The tradeoff: SPL structures are faster for their specific use cases but less flexible than arrays and have an unfamiliar API. For most web applications, native PHP arrays are sufficient — consider SPL structures only when profiling shows array operations as bottlenecks.