php: quick sort algorithm implemented in php

quicksort.php

<?php

$data = array(55,41,59,26,53,58,97,93);

quicksort(0, sizeof($data)-1);

function quicksort($lower, $upper) {
	global $data;
	if ($lower >= $upper) {
		return;
	}

	$m = $lower;

	echo str_repeat("=", 80), PHP_EOL;

	// partition the array on $data[$lower]
	for ($i=$lower+1; $i<=$upper; $i++) {
		if ($data[$i] < $data[$lower]) {
			$tmp = $data[++$m];
			$data[$m] = $data[$i];
			$data[$i] = $tmp;
			print_array($data);
		}
	}

	// swap the sential node with $data[$m]
	$tmp = $data[$m];
	$data[$m] = $data[$lower];
	$data[$lower] = $tmp;
	print_array($data);

	// quick sort the tow parts recursively
	quicksort($lower, $m-1);
	quicksort($m+1, $upper);
}

function print_array($data, $newlineCount = 1) {
	echo implode("->", $data), str_repeat(PHP_EOL, $newlineCount);
}