How is the PHP array implemented at the C level?

PHP Array Implementation on the C Level

The PHP array is a fundamental data structure in PHP, boasting versatility and efficient performance. However, some array functions exhibit slower speeds than expected, leading to the question: How is the PHP array implemented at the C level?

Delving into the PHP core, specifically zend/zend_hash.h and ext/standard/array.c, reveals that the PHP array employs a chained hash table. This structure provides constant-time lookup (O(c)) and handles key collisions through linear search (O(n)). The hashing algorithm accommodates both integer and string keys within the same key space.

Each stored value in the hash links to its preceding and succeeding values, creating a linked list. Additionally, a temporary pointer tracks the current item for seamless iteration.

Regarding array_rand, its inherent randomness dictates iterating over the array randomly (O(n)) to ensure a truly random key. This is due to potential missing keys in the range, making direct key access (O(c)) impossible.

Furthermore, array_key_exists and in_array differ in implementation. array_key_exists utilizes hash lookup, resulting in O(c) performance, while in_array resorts to linear search (O(n)), which becomes inefficient for large arrays.

In summary, the PHP array offers efficient hash-based lookup. However, its linked list structure impacts scalar array operations such as array_rand, particularly noticeable with large arrays. The absence of a clear flag for array creation using array subscript or array_push that would enable C array-like scaling presents an opportunity for potential performance improvements in specific scenarios.

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