$str = "aa:1,bb:2,cc:3,dd:4";
parse_str(preg_replace(array('/\w+/', '/:/','/,/'), array('"$0"','=','&'), $str), $ar);
print_r($ar);

------解决方案--------------------
再来二种
PHP code
$str = "aa:1,bb:2,cc:3,dd:4";
foreach(array_chunk(preg_split('/[:,]/', $str), 2) as $t) $ar[$t[0]] = $t[1];
print_r($ar);

------解决方案--------------------
添块砖

PHP code

$str = "aa:1,bb:2,cc:3,dd:4";
preg_replace("#([\w]+):([\d]+)#e", "\$arr['\\1']=\\2\\2", $str);

print_r($arr);
/*
Array
(
    [aa] => 11
    [bb] => 22
    [cc] => 33
    [dd] => 44
)

*/

------解决方案--------------------
PHP code
[User:root Time:14:14:44 Path:/home/liangdong/php]$ php arr.php 
1
2
3
4
[User:root Time:14:14:45 Path:/home/liangdong/php]$ cat arr.php 
<?php
$str = "aa:1,bb:2,cc:3,dd:4";
$arr = explode(',', $str);
foreach ($arr as $val) {
        $fields = explode(':', $val);
        $key = trim($fields[0]);
        $val = trim($fields[1]);
        $res[$key] = $val;
}
extract($res);
for ($ch = ord('a'); $ch <= ord('d'); ++ $ch) {
        $key = str_repeat(chr($ch), 2);
        echo $$key . PHP_EOL;
}
?>