php - Sorting an array of SimpleXML objects

Question

I've read what I've found on Stackoverflow and am still unclear on this.

I have an array of SimpleXML objects something like this:

array(2) {
  [0]=>
  object(SimpleXMLElement)#2 (2) {
    ["name"]=>
    string(15) "Andrew"
    ["age"]=>
    string(2) "21"
  }
  [1]=>
  object(SimpleXMLElement)#3 (2) {
    ["name"]=>
    string(12) "Beth"
    ["age"]=>
    string(2) "56"
  }
}

And I want to be able to sort by whatever column, ascending or descending. Something like:

sort($data, 'name', 'asc');

Where I can pass in the above array of objects and sort by the value of whichever key I like.

For reference, a similar .NET solution would be along these lines:

XmlSortOrder order = XmlSortOrder.Ascending;
    if ( sortDirection == "asc" ) {
        order = XmlSortOrder.Ascending;
    }
    expression.AddSort( columnSortingOn + "/text()", order, 
        XmlCaseOrder.UpperFirst, "en-us", XmlDataType.Text ); 

I've seen people say

"Use usort"

Followed by a basic example from the PHP manual but this doesn't really explain it. At least not to me. I've also seen people suggest using an external library like SimpleDOM but I want to avoid using something external for this (seemingly, though I cannot yet solve it) little thing.

Any help is appreciated, Thanks!

Answer 1

The usort function allows to you tell PHP

Hey, you! Sort this array I'm giving you with this function I wrote.

It has nothing specifically to do with SimpleXML. It's a generic function for sorting PHP built-in array data collection.

You need to write a function, instance method, or static method to sort the array. The second argument to usort accepts a PHP Callback, which is a pseudo-type that lets you specify which function, instance method, or static method.

The function you write will accept two arguments. These will be two different values from your array

function cmp($a, $b)
{
            if ($a == $b) {
                return 0;
            }
            if($a < $b) {
                return -1;
            }
            if($a > $b) {
                return 1;
            }
}

You need to write this function to return one of three values.

If $a == $b, return 0
If $a > $b, return -1
if $a > $v, return 1  

When you call usort, PHP will run through your array, calling your sorting function/method (in this case cmp over and over again until the array is sorted. In your example, $a and $b will be SimpleXML objects.