Sort rows of a 2d array to always have a specified value as the first element

Question

I have a very big array. Arrays within arrays. Below is a small portion of it;

[lta/] => Array(
    [2012-12-31/] => Array(
        [0] => 31_december_2012.pdf
        [1] => 31_december_2012.xls
        [2] => key_points.html
    )
    ...
)

What I need to do, is get the "key_points.html" value to always start at the top of it's array. Example;

[2012-12-31/] => Array
    (
        [2] => key_points.html            
        [0] => 31_december_2012.pdf
        [1] => 31_december_2012.xls  
    )
    ...
)

I cannot do a simple asort because I never know at which point "key_points.html" is going to appear in the array.

I tried to rename the values "key_points.html" with a view to sorting it and then un-renaming it after;

foreach ($the_array as $array_object => $array_item) {
    if ($array_item == "key_points.html") {
        $array_item = "0001_key_points.html";
    }
}

But that literally seemed to have no effect! it didn't even rename my value. I also tried the same thing with string replace;

$the_array = str_replace("key_points.html", "0001_key_points.html", $the_array);

Is there a function perhaps that allows you to specify a string, and move that to the top of each array each time if it finds it?

Answer 1

Use uasort to specify a custom comparator callback:

uasort($array, function($a, $b) {
  if($a == 'key_points.html') return -1; // Smaller than all
  if($b == 'key_points.html') return 1;  // Larger than all
  return ($a < $b) ? -1 : 1;             // Default sorting
});

Syntax is assuming an up to date PHP (5.3+) with support for anonymous functions.

Answer 2

Use custom function

  function customSort (&$array, $key) {
        $sorter=array();
        $ret=array();
        reset($array);
        foreach ($array as $ii => $val) {
            $sorter[$ii]=$va[$key];
        }
        asort($sorter);
        foreach ($sorter as $ii => $val) {
            $ret[$ii]=$array[$ii];
        }
        $array=$ret;
    }
    customSort($your_array,"2");

Answer 3

It might be a commonly considered solution to make iterated calls of uasort(), but this will not be optimized for performance (which may be a concern for a "big array"). It will work, but I probably wouldn't use it because it is running a relatively expensive sorting algorithm on each row.

Code: (Demo)

array_walk(
    $array['lta'],
    fn(&$v) => uasort(
        $v,
        fn($a, $b) => ($b === 'key_points.html')
            <=>
            ($a === 'key_points.html')
    )
);
var_export($array);

---

Instead, it will be less "work" to search for the key of the sticky element, then append the row to a new declaration of the sticky element. The union operator (+) will not allow the original sticky value to exist late in the row.

Code: (Demo)

foreach ($array['lta'] as &$row) {
    if (($k = array_search('key_points.html', $row)) !== false) {
        $row = [$k => 'key_points.html'] + $row;
    }
}
var_export($array);