In Insertion Sort ,If it is the first element, it is already sorted.Pick next element and compare with all other elements.And Shift all the elements in the sorted sub-list that is greater than the value to be sorted.Repeat the process untill sorted.

<?php
 
    $unsorted_numbers = array(3,7,1,26,43,12,6,21,23,73);
 
    function insertionsort($array)
    {
        
        for ($i = 0; $i < count($array); $i++) {
               $val = $array[$i];
		       $j = $i-1;
		while($j>=0 && $array[$j] > $val){
			$array[$j+1] = $array[$j];
			$j--;
		}
		$array[$j+1] = $val;
        }
 
        return $array;
    }
 
    $sorted_numbers = insertionsort($unsorted_numbers);
 
    print_r($sorted_numbers);

Array
(
    [0] => 1
    [1] => 3
    [2] => 6
    [3] => 7
    [4] => 12
    [5] => 21
    [6] => 23
    [7] => 26
    [8] => 43
    [9] => 73
)