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PHP Associative Arrays

Keeping track of array elements by index works just fine, but can require extra work in terms of remembering which number refers to which product. It can also make code hard for other programmers to follow.
This is where associative arrays come into their own. Using them, you can reference the items in an array by name rather than by number. Example 6-4 expands on the previous code by giving each element in the array an identifying name and a longer, more explanatory string value

Example 6-4. Adding items to an associative array and retrieving them

<?php
$paper['copier'] = "Copier & Multipurpose";
$paper['inkjet'] = "Inkjet Printer";
$paper['laser'] = "Laser Printer";
$paper['photo'] = "Photographic Paper";
echo $paper['laser'];
?>

 

In place of a number (which doesn’t convey any useful information, aside from the position of the item in the array), each item now has a unique name that you can use to reference it elsewhere, as with the echo statement—which simply prints out Laser Printer. The names (copier, inkjet, etc.) are called indexes or keys and the items assigned to them (such as “Laser Printer”) are called values.

This very powerful feature of PHP is often used when you are extracting information from XML and HTML. For example, an HTML parser such as those used by a search engine could place all the elements of a web page into an associative array whose names reflect the page’s structure:

$html['title'] = "My web page";
 $html['body'] = "... the body of the web page ...";

The program would also probably break down all the links found within a page into another array, and all the headings and subheadings into another. When you use associative rather than numeric arrays, the code to refer to all of these items is easy to write and debug

Assignment Using the array Keyword

So far, you’ve seen how to assign values to arrays by just adding new items one at a time. Whether you specify keys, specify numeric identifiers, or let PHP assign numeric identifiers implicitly, this is a long-winded approach. A more compact and faster assignment method uses the array keyword. Example 6-5 shows both a numeric and an associative array assigned using this method.

Example 6-5. Adding items to an array using the array keyword

<?php
$p1 = array("Copier", "Inkjet", "Laser", "Photo");

echo "p1 element: " . $p1[2] . "<br>";

$p2 = array('copier' => "Copier & Multipurpose",
'inkjet' => "Inkjet Printer",
'laser' => "Laser Printer",
'photo' => "Photographic Paper");
echo "p2 element: " . $p2['inkjet'] . "<br>";
?>

The first half of this snippet assigns the old, shortened product descriptions to the array $p1. There are four items, so they will occupy slots 0 through 3. Therefore, the echo statement prints out the following:

p1 element: Laser

The second half assigns associative identifiers and accompanying longer product descriptions to the array $p2 using the format index => value. The use of => is similar to the regular = assignment operator, except that you are assigning a value to an index and not to a variable. The index is then inextricably linked with that value unless it is assigned a new value. The echo command, therefore, prints out:

p2 element: Inkjet Printer

You can verify that $p1 and $p2 are different types of array, because of both of the following
commands, when appended to the code, will cause an Undefined index or Undefined
offset error, as the array identifier for each, is incorrect:

echo $p1['inkjet']; // Undefined index
echo $p2[3]; // Undefined offset

 

Learn Loops for more detail about an associative array. Especially, foreach loop.

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