What are PHP array functions?

PHP has an extensive set of built-in array functions. Essential ones: count($arr) (number of elements), array_push($arr, $val) / array_pop($arr) (add/remove from end), array_shift($arr) / array_unshift($arr, $val) (remove/add at start), in_array($val, $arr) (check if value exists), array_key_exists($key, $arr) (check if key exists), array_merge($a, $b) (merge arrays), array_slice($arr, $offset, $length) (extract portion), array_search($val, $arr) (returns key of value), sort() / rsort() / asort() / ksort() (sorting), array_unique($arr) (remove duplicates), array_flip($arr) (swap keys and values), and array_map() / array_filter() / array_reduce() (functional operations).