gistfile1.php

<?php
function array_slide(&$array, $key, $amount) {
    if ($amount === 0) {
        return;
    }

    $keys   = array_keys($array);
    $values = array_values($array);
    $target = array($keys[$key] => $values[$key]);
    $left = array_slice($array, 0, $key);
    $right = array_slice($array, $key + 1);
    
    if ($amount > 0) {
        if ($amount > count($right)) {
            $amount = count($right);
        }
        $right_left = array_slice($right, 0, $amount);
        $right_right = array_slice($right, $amount);
        $right = $right_left + $target + $right_right;
    } else ($amount < 0) {
        $amount = $key + $amount;
        if ($amount < 0) {
            $amount = 0;
        }
        $left_left = array_slice($left, 0, $amount);
        $left_right = array_slice($left, $amount);
        $left = $left_left + $target + $left_right;
    } 

    $array = $left + $right;
}