What is array

An arrayin PHP is a collection of key/value pairs.
 This means that it maps keys or indexes to values.
Array indexescan be either integers or strings
whereas values can be of any type.
Arrays in PHP are implemented using hash tables, which
 means that accessing a value has an average complexity.

array()construct Arrays can be declared using the array()language
 construct, which generally takes the following form elements inside
square brackets, [], are optional:

array([key =>] value, [key =>] value, ...)

The key is optional, and when it’s not specified, the key is
 automatically assigned one more than the largest previous
 integer key starting with 0. You can intermix the use with
 and without the key even within the same declaration.
The value itself can be of any PHP type, including an array.
 Arrays containing arrays give a similar result as multi-dimensional
 arrays in other languages.

Accessing Array Elements Array elements can be accessed by using
the $arr[key]notation, where keyis either an integer or string expression.
When using a constant string for key,make sure you don’t forget the single or
double quotes, such as $arr["key"]. This notation can be used for both reading
array elements and modifying or creating new elements.

$arr1 = array(1, 2, 3);
$arr2[0] = 1;
$arr2[1] = 2;
$arr2[2] = 3;

The print_r()function has not been covered yet in this book, but when it
is passed an array, it prints out the array’s contents in a readable way. You can
use this function when debugging your scripts.

[0] => 1
[1] => 2
[2] => 3
[0] => 1
[1] => 2
[2] => 3
So, you can see that you can use both the array()construct
 and the $arr[key] notation to create arrays.
Usually, array()is used to declare arrays
whose elements are known at compile-time, and
 the $arr[key]notation is used when the elements
 are only computed at runtime.
PHP also supports a special notation, $arr[], where the key is
 not specified. When creating new array offsets using
 this notation , the key is automatically assigned as one
 more than the largest previous integer key.

Therefore, the previous example can be rewritten as follows:
$arr1 = array(1, 2, 3);
$arr2[] = 1;
$arr2[] = 2;
$arr2[] = 3;

$arr1 = array("name" => "John", "age" => 28);
$arr2["name"] = "John";
$arr2["age"] = 28;
if ($arr1 == $arr2) {
print '$arr1 and $arr2 are the same' . "\n";

Reading array values You can use the
 $arr[key]notation to read array values.
The next few examples build on top of the previous example:
print $arr2["name"];
if ($arr2["age"] < 35) {
print " is quite young\n";
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