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 about those structures, we refer you to external literature about this broad topic.
A key is either a nonnegative integer or a string. If a key is the standard representation of a non-negative integer, it will be interpreted as such (i.e. '8' will be interpreted as 8, while '08' will be interpreted as '08').
A value can be anything.
Omitting keys. If you omit a key, the maximum of the integer-indices is taken, and the new key will be that maximum + 1. 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.
You can also modify an existing array, by explicitly setting values.
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.
The foreach control structure exists specificly for arrays. It provides an easy way to traverse an array.
You might have seen the following syntax in old scripts: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, and then you get in trouble. For example, you already cannot use the words empty and default this way, since they are special keywords.
And, if these arguments don't help: this syntax is simply deprecated, and it might stop working some day.
Tip: When you turn error_reporting to E_ALL, you will see that PHP generates warnings whenever this construct is used. This is also valid for other deprecated 'features'. (put the line error_reporting(E_ALL); in your script)
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 6-4. 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.
Example 6-7. One-based index
Arrays are ordered. You can also change the order using various sorting-functions. See array-functions for more information.
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.