，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

$a = '36,0.00,0.00,100<br><font color='#FF8000'>------解决思路----------------------</font><br>37,0.00,0.00,100<br><font color='#FF8000'>------解决思路----------------------</font><br>38,0.00,0.00,100<br><font color='#FF8000'>------解决思路----------------------</font><br>39,0.00,0.00,100';<br />$b='38';<br />$c='85';<br /><br />echo preg_replace_callback("/(^<br><font color='#FF8000'>------解决思路----------------------</font><br>\<br><font color='#FF8000'>------解决思路----------------------</font><br>$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