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Sorting SimpleXMLElement Object arrays

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How annoying is it when you import data from an XML file (using simplexml_load_string for example) and get what looks like a normal array, but turns out to be a SimpleXMLElement Object which doesn't accept most of the usual array functions.

Sorting SimpleXMLElement Objects

After importing an XML file into PHP we find ourselves with the following structure. The original XML being a series of <result> nodes under a single parent node. A very common structure.

At this stage we can see the data using print_r or similar function which can loop through the SimpleXMLElement Object structure:

SimpleXMLElement Object ( [result] => Array ( [0] => SimpleXMLElement Object ( [zip] => 90210 [city] => Los Angeles [state] => CA ) [1] => SimpleXMLElement Object ( [zip] => 90213 [city] => Beverly Hills [state] => CA ) [2] => SimpleXMLElement Object ( [zip] => 92130 [city] => San Diego [state] => CA ) [3] => SimpleXMLElement Object ( [zip] => 89165 [city] => Las Vegas [state] => NV ) [4] => SimpleXMLElement Object ( [zip] => 85034 [city] => Phoenix [state] => AZ ) ) )

While it looks like $xml->result might be a normal array, trying to apply the usual array functions will not work. Even var_dump will not show you past the first entry as the data for SimpleXMLElement Objects is stored internally by PHP.

Creating a proxy array for sorting

What we can do, however, is create a 'proxy' array for sorting by looping through the results and collecting each node:

<?PHP $sortable = array(); foreach($xml->result as $node) { $sortable[] = $node; } ?>

Now we have a normal PHP Array containing SimpleXMLElement Object nodes:

Array ( [0] => SimpleXMLElement Object ( [zip] => 90210 [city] => Los Angeles [state] => CA ) [1] => SimpleXMLElement Object ( [zip] => 90213 [city] => Beverly Hills [state] => CA ) [2] => SimpleXMLElement Object ( [zip] => 92130 [city] => San Diego [state] => CA ) [3] => SimpleXMLElement Object ( [zip] => 89165 [city] => Las Vegas [state] => NV ) [4] => SimpleXMLElement Object ( [zip] => 85034 [city] => Phoenix [state] => AZ ) )

The reason for PHP keeping SimpleXmlElement data in memory is that it can become very large in which case extracting it into other PHP constructs will consume a lot of memory. This option is best then for small datasets.

Applying the sort

Our Array can now be sorted using a variation of our associative array sorting functions. For example, to sort the list by state and then by city we use the following:

<PHP function compare_city($a, $b) { // sort by state $retval = strnatcmp($a->state, $b->state); // if identical, sort by city if(!$retval) $retval = strnatcmp($a->city, $b->city); return $retval; } // sort alphabetically by state and city usort($sortable, __NAMESPACE__ . '\compare_city'); ?>

The only real difference between this case and a normal associative array sorting function is that we reference the node elements using $object->city instead of $array['city']. The result is as you would expect:

Array ( [0] => SimpleXMLElement Object ( [zip] => 85034 [city] => Phoenix [state] => AZ ) [1] => SimpleXMLElement Object ( [zip] => 90213 [city] => Beverly Hills [state] => CA ) [2] => SimpleXMLElement Object ( [zip] => 90210 [city] => Los Angeles [state] => CA ) [3] => SimpleXMLElement Object ( [zip] => 92130 [city] => San Diego [state] => CA ) [4] => SimpleXMLElement Object ( [zip] => 89165 [city] => Las Vegas [state] => NV ) )

And we're there. Check out the Related Articles links below for more on sorting of arrays and associative arrays.

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