，PHP替换指定位置的字符串

在线等，急。PHP替换指定位置的字符串

$a = '36,0.00,0.00,100|37,0.00,0.00,100|38,0.00,0.00,100|39,0.00,0.00,100';
$b='38';
$c='85';

用PHP怎么做一替换。。。

就是说 当$a中 包含$b中的字数后。 把38，0.00，0.00，100 中的100替换成$c中的数字。

如果$b=‘37’

那么就把37，0.00，0.00，100 中的100替换成$c中的数字。


谢谢各位了
------解决思路----------------------
php5.3+ 通过

$a = '36,0.00,0.00,100
------解决思路----------------------
37,0.00,0.00,100
------解决思路----------------------
38,0.00,0.00,100
------解决思路----------------------
39,0.00,0.00,100';
$b='38';
$c='85';

echo preg_replace_callback("/(^
------解决思路----------------------
\
------解决思路----------------------
$b,0.00,0.00,)100/", function($m) use ($c){ return $m[1].$c;}, $a);

36,0.00,0.00,100
------解决思路----------------------
37,0.00,0.00,100
------解决思路----------------------
38,0.00,0.00,85
------解决思路----------------------
39,0.00,0.00,100