PHP's array_slice vs Python's splitting arrays

Some background

I was having a go at the common "MaxProfit" programming challenge. It basically goes like this:

Given a zero-indexed array A consisting of N integers containing daily prices of a stock share for a period of N consecutive days, returns the maximum possible profit from one transaction during this period.

I was quite pleased with this PHP algorithm I came up, having avoided the naive brute-force attempt:

public function maxProfit($prices)
{
    $maxProfit = 0;
    $key = 0;
    $n = count($prices);

    while ($key < $n - 1) {
        $buyPrice = $prices[$key];
        $maxFuturePrice = max( array_slice($prices, $key+1) );          
        $profit = $maxFuturePrice - $buyPrice;

        if ($profit > $maxProfit) $maxProfit = $profit;
        $key++;
    }
    return $maxProfit;
}

However, having tested my solution it seems to perform badly performance-wise, perhaps even in O(n²) time.

I did a bit of reading around the subject and discovered a very similar python solution. Python has some quite handy array abilities which allow splitting an array with a a[s : e] syntax, unlike in PHP where I used the array_slice function. I decided this must be the bottleneck so I did some tests:

Tests

PHP array_slice()

$n = 10000;    
$a = range(0,$n);

$start = microtime(1);
foreach ($a as $key => $elem) {
    $subArray = array_slice($a, $key);
}
$end = microtime(1);

echo sprintf("Time taken: %sms", round(1000 * ($end - $start), 4)) . PHP_EOL;

Results:

$ php phpSlice.php
Time taken: 4473.9199ms
Time taken: 4474.633ms
Time taken: 4499.434ms

Python a[s : e]

import time

n = 10000
a = range(0, n)

start = time.time()
for key, elem in enumerate(a):
    subArray = a[key : ]
end = time.time()

print "Time taken: {0}ms".format(round(1000 * (end - start), 4))

Results:

$ python pySlice.py 
Time taken: 213.202ms
Time taken: 212.198ms
Time taken: 215.7381ms
Time taken: 213.8121ms

Question

1. Why is PHP's array_slice() around 20x less efficient than Python?
2. Is there an equivalently efficient method in PHP that achieves the above and thus hopefully makes my maxProfit algorithm run in O(N) time?