Question

If I am creating an array with several hundred indexes, how much overhead is required for the additional array versus adding a second dimension to an existing array?

I.E. $pages[‘page’][‘url’] and $titles[‘page’][‘title’] versus $pages[‘page’][‘url’][‘title’] versus $pages[‘page’][‘title’], where the last example assumes each index of $pages contains an associative array.

The goal is to be able to be to lookup two attributes of each ‘page’. Each ‘page’ has a ‘url’ and a ‘title’, which would be more efficient for memory usage on the arrays? Which for access/storing the data?

Two associative arrays:

// Store the information in arrays
$titles['page1'] = 'Page 1 Title';
$titles['page2'] = 'Page 2 Title';

$urls['page1'] = 'http://www.page1.com';
$urls['page2'] = 'http://www.page2.com';

// Display an example
echo $titles['page1'] . ' is at ' . $urls['page1']; 

or one array of arrays:

$pages['page1'] = array( 'title' => 'Page 1 Title', 'url' => 'http://www.page1.com' );
$pages['page2'] = array( 'title' => 'Page 2 Title', 'url' => 'http://www.page2.com' );

// Display an example
echo $pages['page1']['title'] . ' is at ' . $pages['page1']['url'];

Answer 1

The memory usage of a two dimensional array is slightly bigger, but this may result from bad code (?).

<!--
    http://php.net/manual/en/function.memory-get-usage.php

    array1: 116408 (1D 1001 keys)     (1x 100.000 keys:  11.724.512)
    array2: 116552 (2D, 1002 keys)    (2x  50.000 keys:  11.724.768)

    total:    +144                                             +256
-->

<?php

function genRandomString() {
    $length = 10;
    $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
    $string = '';    

    for ($p = 0; $p < $length; $p++) {
        $string .= $characters[mt_rand(0, strlen($characters))];
    }

    return $string;
}

$i = 0;

// ----- Larger 1D array -----
$mem_start = memory_get_usage();
echo "initial1: " . $mem_start . "<br />";
$arr = array();

for ($i = 1; $i <= 1000; $i++) {
    $arr[ genRandomString() ] = "Hello world!";
}

echo "array1: " . (memory_get_usage() - $mem_start) . "<br />";
unset($arr);

// ----- 2D array -----
$mem_start = memory_get_usage();
$arr2 = array();

echo "initial2: " . $mem_start . "<br />"; 


for ($i = 1; $i <= 500; $i++) {
    $arr2["key______1"][ genRandomString() ] = "Hello world!";
    $arr2["key______2"][ genRandomString() ] = "Hello world!";
}

echo "array2: " . (memory_get_usage() - $mem_start) . "<br />";

unset($arr2);

unset($i);
unset($mem_start)    
echo "final: " . memory_get_usage() . "<br />"; 

?>