minmax_sentinel.php

<?php

function min_and_max($list) 
{
	$len = count($list);
	$minimum = $maximum = $list[0];
	
	// adding a centinel
	if ($len % 2 == 0) {
		$list[] = $list[0];
	}

	for ($i = 1; $i < $len; $i+=2) {
		
		if ($list[$i] < $list[$i+1]) { 
		  if ($minimum > $list[$i]) {
				$minimum = $list[$i];
			}
			if ($maximum > $list[$i+1]) { 
			  $maximum = $list[$i+1]; 
			} 
		} else { 
		  if ($minimum > $list[$i+1]) {
				$minimum = $list[$i+1];
			}
			if ($maximum < $list[$i]) { 
			  $maximum = $list[$i]; 
			} 
		} 
	} 
	
	return array('minimum' => $minimum, 'maximum' => $maximum);
}

min_and_max($list);