array_merge() / array_combine() Since: PHP 4（2000）

// Merges multiple arrays into one.
array_merge(array1, array2, ...);

// Creates an associative array from a keys array and a values array.
array_combine(keys_array, values_array);

// Splits an array into chunks of a specified size.
array_chunk(array, size, preserve_keys);

<?php
// Merge two arrays into one.
$front = ['HTML', 'CSS', 'JavaScript'];
$back = ['PHP', 'Python', 'Ruby'];
$all = array_merge($front, $back);
print_r($all); // All 6 elements are combined into a single array.

// When merging associative arrays, duplicate keys are overwritten by the later value.
$defaults = ['color' => 'white', 'size' => 'M', 'stock' => 10];
$custom = ['color' => 'red', 'stock' => 5];
$result = array_merge($defaults, $custom);
print_r($result); // 'color' is overwritten with 'red', 'stock' with 5.

// Create an associative array from a keys array and a values array.
$names = ['Taro', 'Hanako', 'Jiro'];
$ages = [25, 30, 20];
$users = array_combine($names, $ages);
print_r($users); // Results in an associative array keyed by name.

// Split an array into chunks of 3.
$numbers = [1, 2, 3, 4, 5, 6, 7];
$chunks = array_chunk($numbers, 3);
print_r($chunks); // Results in a two-dimensional array with up to 3 elements each.

array_merge() is the most commonly used array merging function. When duplicate string keys exist in associative arrays, the later array's value overwrites the earlier one — making it useful for patterns like applying user settings on top of defaults. For numeric keys, values are never overwritten; all elements are appended and re-indexed starting from zero.

In PHP 7.4 and later, you can also merge arrays using the spread operator: [...$array1, ...$array2]. This syntax can be more intuitive and readable in some cases.

array_combine() requires both arrays to have the same number of elements; a ValueError is thrown if they differ. A common use case is processing CSV data by using the header row as keys and a data row as values.

array_chunk() is handy for implementing pagination or batch processing where you need to handle data in fixed-size groups. For retrieving array keys and values, see array_keys() / array_values().