<?PHP
function bignumber($text) {
	$new = '';
	$i = explode(' ',trim($text));
	foreach( $i as $value) {
		$j = explode(',',$value);		
		$j1 = $j[0];
		$j2 = $j[1];
		unset($j[0], $j[1]);
		array_unshift($j, $j2, $j1);
		$new.= implode(',', $j).' ';
	}
	
	return trim($new);
}
$content = 'a1,a2,a3 b1,b2,b3 c1,c2,c3,c4 ';
echo bignumber($content);
?>