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.
Note:
A short array syntax exists which replaces array[] with [].
Example #1 A simple array
The key can either be an int or a string. The value can be of any type.
Additionally the following key casts will occur:
- Strings containing valid decimal ints, unless the number is preceded by a + sign, will be cast to the int 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 ints, 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 ints, 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 objects can 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
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 int and string keys at the same time as PHP does not distinguish between indexed and associative arrays.
Example #3 Mixed int and string keys
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 int key.
Example #4 Indexed arrays without key
The above example will output:
array[4] { [0]=> string[3] "foo" [1]=> string[3] "bar" [2]=> string[5] "hello" [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
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.
Example #6 Complex Type Casting and Overwriting example
This example includes all variations of type casting of keys and overwriting of elements.
The above example will output:
array[7] { [1]=> string[1] "g" [-1]=> string[1] "d" ["01"]=> string[1] "e" ["1.5"]=> string[1] "f" [0]=> string[1] "h" [""]=> string[1] "j" [2]=> string[1] "l" }
Accessing array elements with square bracket syntax
Array elements can be accessed using the array[key] syntax.
Example #7 Accessing array elements
The above example will output:
string[3] "bar" int[24] string[3] "foo"
Note:
Prior to PHP 8.0.0, square brackets and curly braces could be used interchangeably for accessing array elements [e.g. $array[42] and $array{42} would both do the same thing in the example above]. The curly brace syntax was deprecated as of PHP 7.4.0 and no longer supported as of PHP 8.0.0.
Example #8 Array dereferencing
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.
Note:
Array dereferencing a scalar value which is not a string yields null. Prior to PHP 7.4.0, that did not issue an error message. As of PHP 7.4.0, this issues E_NOTICE; as of PHP 8.0.0, this issues E_WARNING.
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 int or string // value may be any value of any typeIf $arr doesn't exist yet or is set to null or false, 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 a variable by a direct assignment.
Note: As of PHP 7.1.0, applying the empty index operator on a string throws a fatal error. Formerly, the string was silently converted to an array.
Note: As of PHP 8.1.0, creating a new array from false value is deprecated. Creating a new array from null and undefined values is still allowed.
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.
Note:
As mentioned above, if no key is specified, the maximum of the existing int indices is taken, and the new key will be that maximum value plus 1 [but at least 0]. If no int 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:
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 ]
Array destructuring
Arrays can be destructured using the [] [as of PHP 7.1.0] or list[] language constructs. These constructs can be used to destructure an array into distinct variables.
Array destructuring can be used in foreach to destructure a multi-dimensional array while iterating over it.
Array elements will be ignored if the variable is not provided. Array destructuring always starts at index 0. As of PHP 7.1.0, associative arrays can be destructured too. This also allows for easier selection of the right element in numerically indexed arrays as the index can be explicitly specified. Array destructuring can be used for easy swapping of two variables. Note: The spread operator [...] is not supported in assignments. 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 [E_WARNING-level as of PHP 8.0.0] will be issued, and the result will be null. 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, the array can be reindexed using the array_values[] function.
The foreach control structure exists specifically for arrays. It provides an easy way to traverse an array. 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:
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]. It works because PHP automatically converts a bare string [an unquoted string which 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. Warning The fallback to treat an undefined constant as bare string issues an error of level E_NOTICE. This has been deprecated as of PHP 7.2.0, and issues an error of level E_WARNING. As of PHP 8.0.0, it has been removed and throws an Error exception. Note: This 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. Useful functions
Array do's and don'ts
Why is $foo[bar] wrong?