PHP Program to Rearrange positive and negative numbers in O(n) time and O(1) extra space

<?php
// A PHP program to put positive numbers  
// at even indexes (0, 2, 4,..) and negative 
// numbers at odd indexes (1, 3, 5, ..) 

// The main function that rearranges elements 
// of given array. It puts positive elements 
// at even indexes (0, 2, ..) and negative
// numbers at odd indexes (1, 3, ..). 
function rearrange(&$arr, $n) 
{ 
    // The following few lines are similar 
    // to partition process of QuickSort. 
    // The idea is to consider 0 as pivot 
    // and divide the array around it. 
    $i = -1; 
    for ($j = 0; $j < $n; $j++) 
    { 
        if ($arr[$j] < 0) 
        { 
            $i++; 
            swap($arr[$i], $arr[$j]); 
        } 
    } 

    // Now all positive numbers are at end and 
    // negative numbers at the beginning of array. 
    // Initialize indexes for starting point of 
    // positive and negative numbers to be swapped 
    $pos = $i + 1;
    $neg = 0; 

    // Increment the negative index by 2 and 
    // positive index by 1, i.e., swap every 
    // alternate negative number with next
    // positive number 
    while ($pos < $n && $neg < $pos && 
                        $arr[$neg] < 0) 
    { 
        swap($arr[$neg], $arr[$pos]); 
        $pos++; 
        $neg += 2; 
    } 
} 

// A utility function to swap two elements 
function swap(&$a, &$b) 
{ 
    $temp = $a; 
    $a = $b; 
    $b = $temp; 
} 

// A utility function to print an array 
function printArray(&$arr, $n) 
{ 
    for ($i = 0; $i < $n; $i++) 
        echo " " . $arr[$i] . " "; 
} 

// Driver Code
$arr = array(-1, 2, -3, 4, 5, 6, -7, 8, 9); 
$n = count($arr); 
rearrange($arr, $n); 
printArray($arr, $n); 

// This code is contributed
// by rathbhupendra
?>