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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.


Specifying with array()

An array can be created by the array() language-construct. It takes a certain number of comma-separated key => value pairs.

array( [key =>] value
     , ...
// key is either string or nonnegative integer
// value can be anything

$arr = array("foo" => "bar", 12 => true);

echo $arr["foo"]; // bar
echo $arr[12];    // 1

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.

$arr = array("somearray" => array(6 => 5, 13 => 9, "a" => 42));

echo $arr["somearray"][6];    // 5
echo $arr["somearray"][13];   // 9
echo $arr["somearray"]["a"];  // 42

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 -5 being 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.

Creating/modifying with square-bracket syntax

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.

$arr[key] = value;
$arr[] = value;
// key is either string or nonnegative integer
// value can be anything
If $arr doesn't exist yet, it will be created. So this is also an alternative way to specify an array. To change a certain value, just assign a new value to an element specified with its key. If you want to remove a key/value pair, you need to unset() it.

$arr = array(5 => 1, 12 => 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

Useful functions

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().

$a = array(1 => 'one', 2 => 'two', 3 => 'three');
/* 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(1 => 'one', 2 =>'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?

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:

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

This is wrong, but it works. Then, why is it wrong? The reason is that this code has an undefined constant (bar) rather than a string ('bar' - notice the quotes), and PHP may in future define constants which, unfortunately for your code, have the same name. It works, because the undefined constant gets converted to a string of the same name automatically for backward compatibility reasons.

More examples to demonstrate this fact:

// Let's show all errors

$arr = array('fruit' => 'apple', 'veggie' => 'carrot');

// Correct
print $arr['fruit'];  // apple
print $arr['veggie']; // carrot

// Incorrect.  This works but also throws a PHP error of
// level E_NOTICE because of an undefined constant named fruit
// Notice: Use of undefined constant fruit - assumed 'fruit' in...
print $arr[fruit];    // apple

// Let's define a constant to demonstrate what's going on.  We
// will assign value 'veggie' to a constant named fruit.

// Notice the difference now
print $arr['fruit'];  // apple
print $arr[fruit];    // carrot

// The following is okay as it's inside a string.  Constants are not
// looked for within strings so no E_NOTICE error here
print "Hello $arr[fruit]";      // Hello apple

// With one exception, braces surrounding arrays within strings
// allows constants to be looked for
print "Hello {$arr[fruit]}";    // Hello carrot
print "Hello {$arr['fruit']}";  // Hello apple

// This will not work, results in a parse error such as:
// Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING'
// This of course applies to using autoglobals in strings as well
print "Hello $arr['fruit']";
print "Hello $_GET['foo']";

// Concatenation is another option
print "Hello " . $arr['fruit']; // Hello apple

When you turn error_reporting() up to show E_NOTICE level errors (such as setting it to E_ALL) then you will see these errors. By default, error_reporting is turned down to not show them.

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:

echo $arr[somefunc($bar)];

This is an example of using a function return value as the array index. PHP also knows about constants, as you may have seen the E_* ones before.

$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";

Note that E_ERROR is also a valid identifier, just like bar in the first example. But the last example is in fact the same as writing:

$error_descriptions[1] = "A fatal error has occured";
$error_descriptions[2] = "PHP issued a warning";
$error_descriptions[8] = "This is just an informal notice";

because E_ERROR equals 1, etc.

As we already explained in the above examples, $foo[bar] still works but is wrong. 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.

So why is it bad then?

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: To reiterate, inside a double-quoted string, it's valid to not surround array indexes with quotes so "$foo[bar]" is valid. See the above examples for details on why as well as the section on variable parsing in strings.

Converting to array

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()

// Array as (property-)map
$map = array( 'version'    => 4,
              'OS'         => 'Linux',
              'lang'       => 'english',
              'short_tags' => true
// strictly numerical keys
$array = array( 7,
// this is the same as array(0 => 7, 1 => 8, ...)

$switching = array(         10, // key = 0
                    5    =>  6,
                    3    =>  7, 
                    'a'  =>  4,
                            11, // key = 6 (maximum of integer-indices was 5)
                    '8'  =>  2, // key = 8 (integer!)
                    '02' => 77, // key = '02'
                    0    => 12  // the value 10 will be overwritten by 12
// empty array
$empty = array();         

Example 7-5. Collection

$colors = array('red', 'blue', 'green', 'yellow');

foreach ($colors as $color) {
    echo "Do you like $color?\n";

/* output:
Do you like red?
Do you like blue?
Do you like green?
Do you like yellow?

Note that it is currently not possible to change the values of the array directly in such a loop. A workaround is the following:

Example 7-6. Collection

foreach ($colors as $key => $color) {
    // won't work:
    //$color = strtoupper($color);
    // works:
    $colors[$key] = strtoupper($color);

/* output:
    [0] => RED
    [1] => BLUE
    [2] => GREEN
    [3] => YELLOW

This example creates a one-based array.

Example 7-7. One-based index

$firstquarter  = array(1 => 'January', 'February', 'March');

/* output:
    [1] => 'January'
    [2] => 'February'
    [3] => 'March'

Example 7-8. Filling an array

// fill an array with all items from a directory
$handle = opendir('.');
while ($file = readdir($handle)) {
    $files[] = $file;

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.

Example 7-9. Sorting array


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

$fruits = array ( "fruits"  => array ( "a" => "orange",
                                       "b" => "banana",
                                       "c" => "apple"
                  "numbers" => array ( 1,
                  "holes"   => array (      "first",
                                       5 => "second",

// Some examples to address values in the array above 
echo $fruits["holes"][5];    // prints "second"
echo $fruits["fruits"]["a"]; // prints "orange"
unset($fruits["holes"][0]);  // remove "first"

// Create a new multi-dimensional array
$juices["apple"]["green"] = "good"; 

You should be aware, that array assignment always involves value copying. You need to use the reference operator to copy an array by reference.

$arr1 = array(2, 3);
$arr2 = $arr1;
$arr2[] = 4; // $arr2 is changed,
             // $arr1 is still array(2,3)
$arr3 = &$arr1;
$arr3[] = 4; // now $arr1 and $arr3 are the same