PHP Array

PHP Array

Use PHP Array as a key & value map

# PHP array contains elements of key/value pair
$a = array('text' => 'string', 10 => 4, 'absent' => TRUE);

echo $a['text'];               # 'string'
echo $a[10];                   # 4
echo $a['absent'];             # 1

echo "$a[text]";               # If the variable is inside double quote "", text does NOT need to be quoted

Accessing PHP Array using index

$a = array('text' => 'string');
echo $a['text'];               # 'string'

define('text', 'constant');
echo $a[text];                 # A common mistake: not the same as $a['text']
                               # text here refer to the global constant (text)

PHP array key

  • Can either be a string or an integer
  • Other numeric type will truncate to an integer

Removing (unset) PHP array elements

unset($a[4]);         # Delete an element of an array
unset($a);            # Delete $a

Use PHP Array as a 1-dimensional array

$a1 = array(2, 1, 0);          # Without a key, key value starts from 0 and increment by 1 each time
                               # array(3) { [0]=> int(2) [1]=> int(1) [2]=> int(0) }
                               # An array of length 3 with key 0, 1 & 2

Append an element to the end of a PHP array

$a1[] = 4;                     # Append an element: array(0=>2, 1=>1, 2=>0, 3=>4)

Create an empty PHP array

$a1 = array();                 # Empty array

Start a PHP array with a specific key value

$months = array(1=>'Jan', 'Feb', 'Mar', 'Apr'); # Key starts from 1

$a2[3] = 9;                    # array(1) { [3]=> int(9) }

Create a PHP array with a range of value

$a = range(0, 5);              # array(0, 1, 2, 3, 4, 5);
$a = range('a', 'z');

Multi-dimensional PHP Array

$a = array("item" => array("text" => "string", 10 => 4, "absent" => TRUE) , 'flag' => TRUE);

echo $a['item']['text'];                # 'string'
echo $a['item'][10];                    # 4
echo $a['item']['absent'];              # 1

PHP Array indexing

When a key is missing ($a[] = 'text'; ), PHP uses the next available integer index

  • Find the maximum integer index (n) used in the array
  • Use n+1 as the next index
    $a = array(1 => '0', 3 => 'a', 'b', 'c', 'flag' => 'b', 20 => 'y', 'z');
    echo $a[3];           # 'a'
    echo $a[4];           # 'b'
    echo $a[5];           # 'c'
    echo $a[21];          # 'z'
    $a[] = 'd';           # Same as $a[22] = 'd' which 22 is the next index value for $a
    $a['option'] = 'e';

PHP Array Key & Value Functions

Return all keys in a PHP array

$a = array(3 => 'a', 'b', 'c');
$k = array_keys($a);

Re-index the array with key starts from 0

$a2 = array_values($a);

Check the presence of a key or a value in a PHP array

Find if a key exists in a PHP array

$b = array_key_exists('key_name', $a);   # TRUE if key exists
$b = isset($a['key_name']);              # TRUE if key exists and the value is not NULL

Find if a specific element exists in a PHP array

$a = array('x', 'y', 'z');
$b = in_array('x', $a);                    # TRUE: 'x' is in $a

Find the key by the PHP array element value

$k = array_search('x', $a);                # Return the key for value 'x'

PHP Array Copy, Replace, Insert & Delete

PHP array assignment (PHP array assignment is copy by value)

$a = array('a', 'b', 'c');
$a2 = $a;
  • A duplicate copy of the array (copy by value) is made
  • Change in one array have no impact on other
    $a = array("original");
    $b = $a;
    $a[0] = "new";   // $b[0] will remain as "original"

To copy PHP array by reference

$a3 = &$a;           # A variable alias. Changes impact both $a and $a3

Return a sub-set of a PHP array

$r = array_slice($a, 1, 2);                # Return an array starting at index 1 with max of 2 elements
                                           # array(2) { [0]=> string(1) "b" [1]=> string(1) "c" }

Merge PHP array elements into sub-array of size 2

$r = array_chunk($a, 2);                   # Merge array elements into sub-array elements of size 2
                                           # array(2) { [0]=> array(2) { [0]=> string(1) "a" [1]=> string(1) "b"}
                                           #            [1]=> array(1) { [0]=> string(1) "c" } }

Remove PHP array elements

$r = array_splice($a, 2, 3);               # Remove max of 3 elements starting at index 2 from $a (in-place)
                                           # The removed elements are stored in $r
                                           # $a: array(2) { [0]=> string(1) "a" [1]=> string(1) "b"}
                                           # $r: array(1) { [0]=> string(1) "c"}

$r = array_splice($a, 2);                  # Remove elements of $a starting at index 2 to the end

$r = array_splice($a, 1, 2, array(4, 5));  # Remove and then insert an array

Pad a PHP array

$r = array_pad($a, 5, 0);                  # Pad the array with 0 to a max of 5 elements

PHP Array Function

Assign array's element to different variables

$a = array(1, 2, 3);
list($v1, $v2, $v3, $v4) = $a;   # Assign $v1=$a[0]; $v2=$a[1]; $v3=$a[2]; $v4=NULL;

Un-pack the returned function's array result to a list of PHP variables

list($v1, $v2, $v3, $v4) = some_func();  # some_func() can return an array like return array(1, 3);

Merge 2 PHP arrays

$r = array_merge($a1, $a2); # Merge 2 array with $a1 elements come first
                            # If an element in $a2 has the same key as $a1, $a2 value will be used

PHP Array difference

$a1 = array(1,2);
$a2 = array(2,3);
$r = array_diff($a1, $a2);       # Return an array containing the difference
                                 # array(1) { [0]=> int(1) }

Count the number of elements in a PHP array

$t = count($a);             # Number of count

Sum all the elements of a PHP array

$total = array_sum($a);     # Sum of array elements
print_r($a1);               # Print the array

Reverse, Flip or Shuffle PHP array elements

$r = array_reverse($a);     # Reverse an array

$r = array_flip($a);        # Flip the key with value

shuffle($a);                # Randomized the order of the elements

Use function call back to filter out PHP array elements

function is_even ($v) {
  if (($v%2)==0) {
  	return TRUE;
  return FALSE;

$a = array(1, 6, 2, 4);
$r = array_filter($a, 'is_even');   # Create another array from $a that has even number only

Create variables from array (or vice versa)

$a = array("v1" => 1, "v2" => 2);          # Create variables from array
extract($a, EXTR_PREFIX_ALL, "value");     # Same as $value_v1=1; $value_v2 = 2;

$v1 = 1;
$v2 = 2;                                   # Opposite of extract
$a = compact('v1', 'v2');                  # array(2) { ["v1"]=> int(1) ["v2"]=> int(2) }

PHP array operation

PHP Array Operation Description
$a1 + $a2 Union
$a1 == $a2 Equal if both array have the same key/value pairs
$a1 === $a2 Equal if both have the same key/value pairs in the same order and of types
!= <> Not ==
$a !== $b Not ===

PHP Array Iteration

Iterate through a PHP array

$months = array(1=>'Jan', 'Feb', 'Mar', 'Apr');
foreach ($months as $month) {
   echo "$month";

Iterate through a PHP array with key & value pair

foreach ($months as $key => $month) {
   echo "$key $month";

Iterate through a PHP array with key & value pair by reference

foreach ($months as &$month) {           # Pass by reference
   $month = strtoupper($month);          # so in-place assignment is possible

without the reference, making change to "$month" will have no impact on the $months array

PHP iterate function

# Reset the iterator to the beginning

# Iterate each key/value in the array
while (list($key, $value) = each($a)) {
      echo $key, $value;
PHP Array Iteration Function Description
current( ) Current iterator element
reset( ) Move back to first element
next( ) Move to next element and return it
prev( ) Move back and return it
end( ) Move to last element and return it
each( ) return current key/value as an array and move to next element
key( ) key of current element

PHP array_walk: Calling a function in visiting every element of a PHP array

function visit($value, $key) {

function visit2($value, $key, $p1) {

$a = array(1, 2, 3);
array_walk($a, 'visit');
array_walk($a, 'visit2', 'some parameter');

PHP Array Aggregate features

function sum ($total, $value) {
  $total += $value;
  return $total;

$a = array(1, 2, 3);
$sum = array_reduce($a, 'sum');    # 6
$sum = array_reduce($a, 'sum', 4); # 10: Start $total with 4

PHP Array Sorting Functions

PHP Array Function Sorts by Maintain key/value association Sorting order
array_multisort() value Not on numeric key first array
asort() value yes ASC
arsort() value yes DESC
krsort() key yes DESC
ksort() key yes ASC
natcasesort() value yes Natural & case insensitive
natsort() value yes Natural
rsort() value no (key restart from 0) DESC
shuffle() value no (key restart from 0) Random
sort() value no (key restart from 0) ASC
uasort() value yes User defined
uksort() key yes User defined
usort() value no (key restart from 0) User defined

PHP user define comparator

function custom_sort($a, $b) {
   if ($a == $b) {
     return 0;
   return (($a < $b) ? -1 : 1);

$a = array (1, 4, 4, 2, 7);
uasort($a, 'custom_sort');

Sort array that contain value with both numeric and string (like 'time3', 'time10')


Sort by $a1, then $a2 and last $a3

array_multisort($a1, SORT_ASC, $a2, SORT_DESC, $a3, SORT_DESC);

PHP Array Functions (Quick Reference)

Functions Description
array_change_key_case Change the case of the keys
array_chunk Split an array into chunks
array_combine Create an array using one array as keys and another for values
array_count_values Count the number of occurrence for unique key
array_diff_assoc Find the difference. Comparing key/value
array_diff_key Find the difference. Only comparing key
array_diff_uassoc Find the difference with user defined comparator on indexes
array_diff_ukey Find the difference with user defined comparator on keys
array_diff Difference on an array
array_fill_keys Fill an array with value/key
array_fill Fill an array with values
array_filter Filters elements of an array with a callback function
array_flip Flip key/value
array_intersect_assoc Intersection of arrays with index check
array_intersect_key Intersection of arrays using keys for comparison
array_intersect_uassoc Intersection of arrays with index check with a callback function
array_intersect_ukey Intersection of arrays using a callback function on the keys
array_intersect Intersection of arrays
array_key_exists Key exists in an array
array_keys Return an array of all keys
array_map Call the callback for all elements in an array
array_merge_recursive Merge two or more arrays recursively
array_merge Merge one or more arrays
array_multisort Sort multiple or multi-dimensional arrays
array_pad Pad array
array_pop Pop the element from an array
array_product Product of values in an array
array_push Push an array
array_rand Pick an element randomly
array_reduce Aggregate the array to a single value using a callback function
array_replace_recursive Replaces elements of first array by another array recursively
array_replace Replaces elements of first array by another array
array_reverse Reverse an array
array_search Search for a given value and return the key
array_shift Shift an element off from the beginning
array_slice Slice an array
array_splice Slice an array and replace it with other elements
array_sum Sum of an array
array_udiff_assoc The difference of arrays with index check & compares value with a callback function
array_udiff_uassoc The difference of arrays with index check, compares key/value with a callback function
array_udiff The difference of arrays comparing value with a callback function
array_uintersect_assoc Intersection of arrays with index check, compares value with a callback function
array_uintersect_uassoc The intersection of arrays with index check, compares key/values with a callback functions
array_uintersect The intersection of arrays, compares value with a callback function
array_unique Removes duplicate values
array_unshift Prepend elements to the beginning of an array
array_values Values of an array
array_walk_recursive Call a user function for each element of an array recursively
array_walk Call a user function for each element of an array
array Create an array
compact Create array with variables/values
count Count elements in an array
extract Import variables from an array
in_array Value exists in an array
list Assign variables from an array
range An array containing a range of values
shuffle Shuffle an array
sizeof Count of an array