How to define an array in PHP

This article introduces several methods of defining arrays in PHP. Friends in need can refer to the need

Array array

The actual array in PHP is an ordered map. A map is a type that associates values ​​to keys. This type is optimized in many ways, so it can be treated as a real array, or list (vector), hash table (which is an implementation of map), dictionary, set, stack, queue and many more possibilities. Since the value of an array element can also be another array, tree structures and multidimensional arrays are also allowed.

Generally speaking, the definition methods are as follows:

Method 1:

$a=array(1,2,4,5,6);

<?php
$array=array(&#39;a&#39;,&#39;b&#39;,&#39;c&#39;);
$array[]=&#39;simon&#39;;
print_r($array);
?>

The running results are as follows.
Array
(
[0]=>a
[1]=>b
[2]=>c
[3]=>simon
)

Method Two:

$a=array(key1=>value1,key2=>value2,key3=>value3);

Method Three:

$a[key1]=value1;
$a[key2]=value2;

Method 4: Define the array through square brackets []

You can write like this after version 5.4 of PHP, with the new array abbreviation syntax.

php versions 5.3 and earlier do not accept writing like this...

$data = [
'start_time' => '123',
'end_time' =>'456'
];

Explaining these structures is beyond the scope of this manual, but for each The structure will at least provide an example. For more information on these structures, it is recommended to consult other works on this broad topic.

Syntax

Define array array()

You can use the array() language structure to create a new array. It accepts any number of comma-separated key => value pairs.

array( key => value
, ...
)
// The key (key) can be an integer or a string string
// Value (value ) can be a value of any type
The comma after the last array element can be omitted. Usually used in single-line array definitions, such as array(1, 2) instead of array(1, 2, ). It is common to leave the last comma in multi-line array definitions to make it easier to add a new cell.

Since 5.4, you can use short array definition syntax, using [] instead of array().

Example #1 A simple array

<?php
$array = array(
  "foo" => "bar",
  "bar" => "foo",
);

// 自 PHP 5.4 起
$array = [
  "foo" => "bar",
  "bar" => "foo",
];
?>

key can be integer or string. value can be of any type.

In addition, key will have the following forced conversion:

Strings containing legal integer values ​​will be converted to integers. For example, the key name "8" will actually be stored as 8. But "08" will not be cast because it is not a legal decimal value.
Floating point numbers will also be converted to integers, which means their decimal parts will be rounded off. For example, the key name 8.7 will actually be stored as 8.
Boolean values ​​will also be converted to integers. That is, the key name true will actually be stored as 1 and the key name false will be stored as 0.
Null will be converted to an empty string, that is, the key name null will actually be stored as "".
Arrays and objects cannot be used as key names. Insisting on doing this will result in a warning: Illegal offset type.
If multiple units use the same key name in the array definition, only the last one will be used, and the previous ones will be overwritten.

Example #2 Type coercion and overwriting example

<?php
$array = array(
  1  => "a",
  "1" => "b",
  1.5 => "c",
  true => "d",
);
var_dump($array);
?>

The above routine will output:

array( 1) {
[1]=>
string(1) "d"
}
In the above example, all key names are forced to be converted to 1, and each new unit will be overwritten. The value of the previous one, and only one "d" is left at the end.

PHP arrays can contain both integer and string type key names, because PHP does not actually distinguish between index arrays and associative arrays.

If no key name is specified for the given value, the current largest integer index value will be taken, and the new key name will be that value plus one. If the specified key name already has a value, the value will be overwritten.

Example #3 Mixing integer and string key names

##

<?php
$array = array(
  "foo" => "bar",
  "bar" => "foo",
  100  => -100,
  -100 => 100,
);
var_dump($array);
?>

The above routine will output:

array (4) {

["foo"]=>
string(3) "bar"
["bar"]=>
string(3) "foo"
[ 100]=>
int(-100)
[-100]=>
int(100)
}
key is optional. If not specified, PHP will automatically use the largest previously used integer key plus 1 as the new key.

Example #4 Index array without key name

<?php
$array = array("foo", "bar", "hallo", "world");
var_dump($array);
?>

The above routine will output:

array(4) {

[0]=>
string(3) "foo"
[1]=>
string(3) "bar"
[2]=>
string( 5) "hallo"
[3]=>
string(5) "world"
}
You can also specify key names only for certain units and leave others blank:

Example #5 Only specify key names for some units

<?php
$array = array(
     "a",
     "b",
  6 => "c",
     "d",
);
var_dump($array);
?>

The above routine will output:

array(4) {
[0]=>
string(1) "a"
[1]=>
string(1) "b"
[6]=>
string(1) "c"
[7]=>
string(1) "d"
}
可以看到最后一个值 "d" 被自动赋予了键名 7。这是由于之前最大的整数键名是 6。

用方括号语法访问数组单元 ¶

数组单元可以通过 array[key] 语法来访问。

Example #6 访问数组单元

<?php
$array = array(
  "foo" => "bar",
  42  => 24,
  "multi" => array(
     "dimensional" => array(
       "array" => "foo"
     )
  )
);

var_dump($array["foo"]);
var_dump($array[42]);
var_dump($array["multi"]["dimensional"]["array"]);
?>

以上例程会输出：

string(3) "bar"
int(24)
string(3) "foo"
Note:
方括号和花括号可以互换使用来访问数组单元（例如 $array[42] 和 $array{42} 在上例中效果相同）。
自 PHP 5.4 起可以用数组间接引用函数或方法调用的结果。之前只能通过一个临时变量。

自 PHP 5.5 起可以用数组间接引用一个数组原型。

Example #7 数组间接引用

<?php
function getArray() {
  return array(1, 2, 3);
}

// on PHP 5.4
$secondElement = getArray()[1];

// previously
$tmp = getArray();
$secondElement = $tmp[1];

// or
list(, $secondElement) = getArray();
?>

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