An array in PHP is actually an ordered map. A map is a type that maps values to keys. This type is optimized in several ways, so you can use it as a real array, or a list (vector), hashtable (which is an implementation of a map), dictionary, collection, stack, queue and probably more. Because you can have another PHP-array as a value, you can also quite easily simulate trees.
Explanation of those structures is beyond the scope of this manual, but you'll find at least one example for each of those structures. For more information we refer you to external literature about this broad topic.
A key is either an integer or a string. If a key is the standard representation of an integer, it will be interpreted as such (i.e. "8" will be interpreted as 8, while "08" will be interpreted as "08"). There are no different indexed and associative array types in PHP, there is only one array type, which can both contain integer and string indices.
A value can be of any PHP type.
If you omit a key, the maximum of the integer-indices is taken, and the new key will be that maximum + 1. As integers can be negative, this is also true for negative indices. Having e.g. the highest index being -6 will result in being -5 the new key. If no integer-indices exist yet, the key will be 0 (zero). If you specify a key that already has a value assigned to it, that value will be overwritten.
// This array is the same as ... array(5 => 43, 32, 56, "b" => 12); // ...this array array(5 => 43, 6 => 32, 7 => 56, "b" => 12);
Using TRUE as a key will evalute to integer 1 as key. Using FALSE as a key will evalute to integer 0 as key. Using NULL as a key will evaluate to an empty string. Using an emptry string as key will create (or overwrite) a key with an empty string and its value, it is not the same as using empty brackets.
You cannot use arrays or objects as keys. Doing so will result in a warning: Illegal offset type.
You can also modify an existing array, by explicitly setting values in it.
This is done by assigning values to the array while specifying the key in brackets. You can also omit the key, add an empty pair of brackets ("") to the variable-name in that case.
There are quite some useful function for working with arrays, see the array functions section.
Note: The unset() function allows unsetting keys of an array. Be aware that the array will NOT be reindexed. If you only use "usual integer indices" (starting from zero, increasing by one), you can achive the reindex effect by using array_values().
The foreach control structure exists specifically for arrays. It provides an easy way to traverse an array.
You should always use quotes around an associative array index. For example, use $foo['bar'] and not $foo[bar]. But why is $foo[bar] wrong? You might have seen the following syntax in old scripts:
As stated in the syntax section, there must be an expression between the square brackets ('[' and ']'). That means that you can write things like this:
$error_descriptions[E_ERROR] = "A fatal error has occured"; $error_descriptions[E_WARNING] = "PHP issued a warning"; $error_descriptions[E_NOTICE] = "This is just an informal notice";
$error_descriptions = "A fatal error has occured"; $error_descriptions = "PHP issued a warning"; $error_descriptions = "This is just an informal notice";
Then, how is it possible that $foo[bar] works? It works, because bar is due to its syntax expected to be a constant expression. However, in this case no constant with the name bar exists. PHP now assumes that you meant bar literally, as the string "bar", but that you forgot to write the quotes.
At some point in the future, the PHP team might want to add another constant or keyword, or you may introduce another constant into your application, and then you get in trouble. For example, you already cannot use the words empty and default this way, since they are special reserved keywords.
Note: When you turn error_reporting to E_ALL, you will see that PHP generates notices whenever an index is used which is not defined. Consider this script:The output is:
<?php // Turn on the display of all errors error_reporting(E_ALL); // Define the test array $abc = array("x" => "y"); // Access element with the *bad* method echo $abc[x]; ?>
For any of the types: integer, float, string, boolean and resource, if you convert a value to an array, you get an array with one element (with index 0), which is the scalar value you started with.
If you convert an object to an array, you get the properties (member variables) of that object as the array's elements. The keys are the member variable names.
If you convert a NULL value to an array, you get an empty array.
The array type in PHP is very versatile, so here will be some examples to show you the full power of arrays.
// this $a = array( 'color' => 'red', 'taste' => 'sweet', 'shape' => 'round', 'name' => 'apple', 4 // key will be 0 ); // is completely equivalent with $a['color'] = 'red'; $a['taste'] = 'sweet'; $a['shape'] = 'round'; $a['name'] = 'apple'; $a = 4; // key will be 0 $b = 'a'; $b = 'b'; $b = 'c'; // will result in the array array(0 => 'a' , 1 => 'b' , 2 => 'c'), // or simply array('a', 'b', 'c')
Example 7-4. Using array()
Note that it is currently not possible to change the values of the array directly in such a loop. A workaround is the following:
This example creates a one-based array.
Arrays are ordered. You can also change the order using various sorting-functions. See the array functions section for more information. You can count the number of items in an array using the count() function.
Because the value of an array can be everything, it can also be another array. This way you can make recursive and multi-dimensional arrays.
Example 7-10. Recursive and multi-dimensional arrays
You should be aware, that array assignment always involves value copying. You need to use the reference operator to copy an array by reference.