5.5.1. Slicing an Array

$subset = array_slice(array, offset, length);

The array_slice( ) function returns a new array consisting of a consecutive series of values from the original array. The offset parameter identifies the initial element to copy (0 represents the first element in the array), and the length parameter identifies the number of values to copy. The new array has consecutive numeric keys starting at 0. For example:

$people = array('Tom', 'Dick', 'Harriet', 'Brenda', 'Jo');
$middle = array_slice($people, 2, 2); // $middle is array('Harriet', 'Brenda')

// this use of array_slice( ) makes no sense
$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Betty');
$subset = array_slice($person, 1, 2);  // $subset is array(0 => 35, 1 => 'Betty')

$order = array('Tom', 'Dick', 'Harriet', 'Brenda', 'Jo');
list($second, $third) = array_slice($order, 1, 2);
// $second is 'Dick', $third is 'Harriet'

5.5.2. Splitting an Array into Chunks

$chunks = array_chunk(array, size [, preserve_keys]);

The function returns an array of the smaller arrays. The third argument, preserve_keys, is a Boolean value that determines whether the elements of the new arrays have the same keys as in the original (useful for associative arrays) or new numeric keys starting from 0 (useful for indexed arrays). The default is to assign new keys, as shown here:

$nums = range(1, 7);
$rows = array_chunk($nums, 3);
print_r($rows);
Array
(
    [0] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )
    [1] => Array
        (
            [0] => 4
            [1] => 5
            [2] => 6
        )
    [2] => Array
        (
            [0] => 7
        )
)

5.5.3. Keys and Values

$array_of_keys = array_keys(array);

Here's an example:

$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
$keys = array_keys($person);      // $keys is array('name', 'age', 'wife')

$array_of_values = array_values(array);

As with array_keys( ), the values are returned in the array's internal order:

$values = array_values($person);       // $values is array('Fred', 35, 'Wilma');

5.5.4. Checking Whether an Element Exists

if (array_key_exists(key, array)) { ... }

The function returns a Boolean value that indicates whether the second argument is a valid key in the array given as the first argument.

It's not sufficient to simply say:

if ($person['name']) { ... }           // this can be misleading

Even if there is an element in the array with the key name, its corresponding value might be false (i.e., 0, NULL, or the empty string). Instead, use array_key_exists( ) as follows:

$person['age'] = 0;                    // unborn?
if ($person['age']) {
  echo "true!\n";
}
if (array_key_exists('age', $person)) {
  echo "exists!\n";
}
exists!

$a = array(0,NULL,'');
function tf($v) { return $v ? "T" : "F"; }
for ($i=0; $i < 4; $i++) {
  printf("%d: %s %s\n", $i, tf(isset($a[$i])), tf(array_key_exists($i, $a)));
}
0: T T
1: F T
2: T T
3: F F

5.5.5. Removing and Inserting Elements in an Array

$removed = array_splice(array, start [, length [, replacement ] ]);

We'll look at array_splice( ) using this array:

$subjects = array('physics', 'chem', 'math', 'bio', 'cs', 'drama', 'classics');

We can remove the math, bio, and cs elements by telling array_splice( ) to start at position 2 and remove 3 elements:

$removed = array_splice($subjects, 2, 3);
// $removed is array('math', 'bio', 'cs')
// $subjects is array('physics', 'chem');

If you omit the length, array_splice( ) removes to the end of the array:

$removed = array_splice($subjects, 2);
// $removed is array('math', 'bio', 'cs', 'drama', 'classics')
// $subjects is array('physics', 'chem');

If you simply want to delete the elements and you don't care about their values, you don't need to assign the results of array_splice( ):

array_splice($subjects, 2);
// $subjects is array('physics', 'chem');

To insert elements where others were removed, use the fourth argument:

$new = array('law', 'business', 'IS');
array_splice($subjects, 4, 3, $new);
// $subjects is array('physics', 'chem', 'math', 'bio', 'law', 'business', 'IS')

The size of the replacement array doesn't have to be the same as the number of elements you delete. The array grows or shrinks as needed:

$new = array('law', 'business', 'IS');
array_splice($subjects, 2, 4, $new);
// $subjects is array('physics', 'chem', 'math', 'law', 'business', 'IS')

To get the effect of inserting new elements into the array, delete zero elements:

$subjects = array('physics', 'chem', 'math');
$new = array('law', 'business');
array_splice($subjects, 2, 0, $new);
// $subjects is array('physics', 'chem', 'law', 'business', 'math')

$capitals = array('USA'           => 'Washington',
                  'Great Britain' => 'London',
                  'New Zealand'   => 'Wellington',
                  'Australia'     => 'Canberra',
                  'Italy'         => 'Rome');
$down_under = array_splice($capitals, 2, 2); // remove New Zealand and Australia
$france = array('France' => 'Paris');
array_splice($capitals, 1, 0, $france);      // insert France between USA and G.B.