Several methods of PHP array definition

Array array

What is an array? Arrays are special variables that can hold more than one value at the same time. An array in PHP is actually 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 PHP version 5.4, 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 at least one example will be provided for each structure. 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
     , ...
     )
// 键（key）可是是一个整数 integer 或字符串 string
// 值（value）可以是任意类型的值

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 it is possible to use the 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 forcing 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"
}

All key names in the above example are forced Convert to 1, then each new cell will overwrite the value of the previous one, leaving only a "d" in 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 some units and leave others blank:

Example #5 Specify key names only 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"

}

You can see that the last value "d" is automatically assigned the key name 7. This is because the previous largest integer key was 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(); ?>

Note:

试图访问一个未定义的数组键名与访问任何未定义变量一样：会导致 E_NOTICE 级别错误信息，其结果为 NULL。

更多关于PHP数组的问题可以访问相关网站：//m.sbmmt.com/search/PHP数组.html

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