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PHP Array - Free Tutorials and scripts


 

Arrays

An array in PHP is actually an ordered map. A map is a type that associates values to keys. This type is optimized for several different uses; it can be treated as an array, list (vector), hash table (an implementation of a map), dictionary, collection, stack, queue, and probably more. As array values can be other arrays, trees and multidimensional arrays are also possible.

Explanation of those data structures is beyond the scope of this manual, but at least one example is provided for each of them. For more information, look towards the considerable literature that exists about this broad topic.

Syntax

Specifying with array()

An array can be created using the array() language construct. It takes any number of comma-separated key => value pairs as arguments.

array(
    key  => value,
    key2 => value2,
    key3 => value3,
    ...
)

The comma after the last array element is optional and can be omitted. This is usually done for single-line arrays, i.e. array(1, 2) is preferred over array(1, 2, ). For multi-line arrays on the other hand the trailing comma is commonly used, as it allows easier addition of new elements at the end.

As of PHP 5.4 you can also use the short array syntax, which replaces array() with [].

Example #1 A simple array

$array = array(
    
"foo" => "bar",
    
"bar" => "foo",
);


// as of PHP 5.4
$array = [
    
"foo" => "bar",
    
"bar" => "foo",
];

?>

The key can either be an integer or a string. The value can be of any type.

Additionally the following key casts will occur:

  • Strings containing valid integers will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.
  • Floats are also cast to integers, which means that the fractional part will be truncated. E.g. the key 8.7 will actually be stored under 8.
  • Bools are cast to integers, too, i.e. the key true will actually be stored under 1 and the key false under 0.
  • Null will be cast to the empty string, i.e. the key null will actually be stored under "".
  • Arrays and objectcan not be used as keys. Doing so will result in a warning: Illegal offset type.

 

If multiple elements in the array declaration use the same key, only the last one will be used as all others are overwritten.

Example #2 Type Casting and Overwriting example

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

var_dump($array);
?>

The above example will output:

array(1) {
  [1]=>
  string(1) "d"
}

As all the keys in the above example are cast to 1, the value will be overwritten on every new element and the last assigned value "d" is the only one left over.

PHP arrays can contain integer and string keys at the same time as PHP does not distinguish between indexed and associative arrays.

Example #3 Mixed integer and string keys

$array = array(
    
"foo" => "bar",
    
"bar" => "foo",
    
100   => -100,
    -
100  => 100,
);

var_dump($array);
?>

The above example will output:

array(4) {
  ["foo"]=>
  string(3) "bar"
  ["bar"]=>
  string(3) "foo"
  [100]=>
  int(-100)
  [-100]=>
  int(100)
}

The key is optional. If it is not specified, PHP will use the increment of the largest previously used integer key.

Example #4 Indexed arrays without key

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

The above example will output:

array(4) {
  [0]=>
  string(3) "foo"
  [1]=>
  string(3) "bar"
  [2]=>
  string(5) "hallo"
  [3]=>
  string(5) "world"
}

It is possible to specify the key only for some elements and leave it out for others:

Example #5 Keys not on all elements

$array = array(
         
"a",
         
"b",
    
=> "c",
         
"d",
);

var_dump($array);
?>

The above example will output:

array(4) {
  [0]=>
  string(1) "a"
  [1]=>
  string(1) "b"
  [6]=>
  string(1) "c"
  [7]=>
  string(1) "d"
}

As you can see the last value "d" was assigned the key 7. This is because the largest integer key before that was 6.

Accessing array elements with square bracket syntax

Array elements can be accessed using the array[key] syntax.

Example #6 Accessing array elements

$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"]);
?>

The above example will output:

string(3) "bar"
int(24)
string(3) "foo"

As of PHP 5.4 it is possible to array dereference the result of a function or method call directly. Before it was only possible using a temporary variable.

Example #7 Array dereferencing


function getArray() {
    return array(
123);
}


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

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

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

Note:

Attempting to access an array key which has not been defined is the same as accessing any other undefined variable: an E_NOTICE-level error message will be issued, and the result will be NULL.

 

Creating/modifying with square bracket syntax

An existing array can be modified by explicitly setting values in it.

This is done by assigning values to the array, specifying the key in brackets. The key can also be omitted, resulting in an empty pair of brackets ([]).

$arr[key] = value;
$arr[] = value;
// key may be an integer or string
// value may be any value of any type

If $arr doesn't exist yet, it will be created, so this is also an alternative way to create an array. This practice is however discouraged because if $arr already contains some value (e.g. string from request variable) then this value will stay in the place and [] may actually stand for string access operator. It is always better to initialize variable by a direct assignment.

To change a certain value, assign a new value to that element using its key. To remove a key/value pair, call the unset() function on it.

$arr = array(=> 112 => 2);

$arr[] = 56;    // This is the same as $arr[13] = 56;
                // at this point of the script


$arr["x"] = 42// This adds a new element to
                // the array with key "x"
                

unset($arr[5]); // This removes the element from the array

unset($arr);    // This deletes the whole array
?>

Note:

As mentioned above, if no key is specified, the maximum of the existing integer indices is taken, and the new key will be that maximum value plus 1 (but at least 0). If no integer indices exist yet, the key will be 0 (zero).

Note that the maximum integer key used for this need not currently exist in the array. It need only have existed in the array at some time since the last time the array was re-indexed. The following example illustrates:


// Create a simple array.
$array = array(12345);
print_r($array);

// Now delete every item, but leave the array itself intact:
foreach ($array as $i => $value) {
    unset(
$array[$i]);
}

print_r($array);

// Append an item (note that the new key is 5, instead of 0).
$array[] = 6;
print_r($array);

// Re-index:
$array array_values($array);
$array[] = 7;
print_r($array);
?>

The above example will output:

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
)
Array
(
)
Array
(
    [5] => 6
)
Array
(
    [0] => 6
    [1] => 7
)

 

Useful functions

There are quite a few useful functions for working with arrays. See the array functions section.

Note:

The unset() function allows removing keys from an array. Be aware that the array will not be reindexed. If a true "remove and shift" behavior is desired, thearray can be reindexed using the array_values() function.

$a = array(=> 'one'=> 'two'=> 'three');
unset(
$a[2]);
/* will produce an array that would have been defined as
   $a = array(1 => 'one', 3 => 'three');
   and NOT
   $a = array(1 => 'one', 2 =>'three');
*/


$b array_values($a);
// Now $b is array(0 => 'one', 1 =>'three')
?>

 

The foreach control structure exists specifically for arrays. It provides an easy way to traverse an array.

Array do's and don'ts

Why is $foo[bar] wrong?

Always use quotes around a string literal array index. For example, $foo['bar'] is correct, while $foo[bar] is not. But why? It is common to encounter this kind of syntax in old scripts:

$foo[bar] = 'enemy';
echo 
$foo[bar];
// etc
?>

This is wrong, but it works. The reason is that this code has an undefined constant (bar) rather than a string ('bar' - notice the quotes). PHP may in future define constants which, unfortunately for such code, have the same name. It works because PHP automatically converts a bare string (an unquoted stringwhich does not correspond to any known symbol) into a string which contains the bare string. For instance, if there is no defined constant named bar, then PHP will substitute in the string 'bar' and use that.

NoteThis does not mean to always quote the key. Do not quote keys which are constants or variables, as this will prevent PHP from interpreting them.

error_reporting(E_ALL);
ini_set('display_errors'true);
ini_set('html_errors'false);