# 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

$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)

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

$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

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

$a1 = array();                 # Empty array

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

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

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

$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

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

$a2 = array_values($a);

$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

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

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

$a = array('a', 'b', 'c');
$a2 = $a;

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

$r = array_slice($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" }

$r = array_chunk($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" } }
                                           #

$r = array_splice($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

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

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

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

$r = array_merge($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

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

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

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

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

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

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

$a = array("v1" => 1, "v2" => 2);          # Create variables from array
extract($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) }

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

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

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

# Reset the iterator to the beginning
reset($a);

# Iterate each key/value in the array
while (list($key, $value) = each($a)) {
      echo $key, $value;
}

function visit($value, $key) {
  ...
}

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

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

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

sort($a);

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');

natsort($a);
natcasesort($a);

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