<?php

$array = array(1,2,3,4,5,6,7);

$element = $array[4];
unset($array[4]);
$array[] = $element;

print_r($array);

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [5] => 6
    [6] => 7
    [7] => 5
)