<?php

$a = array('b' => 0, 'a' => 1, 'c' => 2);

print_r($a);
	
$keys = array_keys($a);

$i = array_search('a', $keys);
$j = array_search('c', $keys);

if ($i !== false && $j !== false) {
	$keys[$i] = 'c';
	$keys[$j] = 'a';
}

$b = array();

foreach ($keys as $key)
	$b[$key] = $a[$key];
	
print_r($b);