$str = 'a,100,b,32,c,10,d,20';
$res_str = implode(',',array_map(create_function('$v', 'return implode("=>",$v);'), array_chunk(explode(',', $str), 2)));
$res =  eval("return array($res_str);");

echo '<pre>';
print_r($res);
echo '</pre>';
/*
Array
(
    [a] => 100
    [b] => 32
    [c] => 10
    [d] => 20
)
*/

------解决方案--------------------
直接的函数没有，恶心的代码有很多...

PHP code

$Str = 'a,100,b,32,c,10';
preg_match_all('/[a-z]+/i', $Str, $Aarray);
$Aarray = $Aarray[0];
preg_match_all('/[0-9]+/', $Str, $Barray);
$Barray = $Barray[0];
$Carray = array_combine($Aarray, $Barray);
print_r($Carray);

------解决方案--------------------
11文 你又来鸟 我只想到 explode
PHP code

$str = 'a,100,b,32,c,10';
$arr = explode(',',$str);
for($i = 0 ; $i <count($arr); $i++)
{
    if($i %2 ==0)
        $new_arr[$arr[$i]] =  $arr[$i+1];
}

------解决方案--------------------
来一个另类的
PHP code
$s = 'a,100,b,32,c,10';

preg_replace('/(\w+),([^,]+)/se', '$ar[$1]=$2', $s);
print_r($ar);