选择排序的PHP实现代码如下：

<?php
    
/**
* 选择排序算法实现
*/
function selection_sort($nums)
{
    if (count($nums) <= 1) {
        return $nums;
    }

    for ($i = 0; $i < count($nums); $i++) {
        $min= $i;
        for ($j = $i + 1; $j < count($nums); $j++) {
            if ($nums[$j] < $nums[$min]) {
                $min = $j;
            }
        }
        if ($min != $i) {
            $temp = $nums[$i];
            $nums[$i] = $nums[$min];
            $nums[$min] = $temp;
        }
    }

    return $nums;
}

$nums = [4, 5, 6, 3, 2, 1];
$nums = selection_sort($nums);
print_r($nums);