How to make a subpattern replace with number directly after?

[ZeratulsDomain]

Here is my code:

$string = '<a href="a.php?id=">sometext</a>';
 $result = (preg_replace('/(.+href=)"(.+\..+)"/i', '$1"$2abc"', $string) );

My goal is to give this url (id=) a value. Using the code above ill get the value "abc". Anytime I change "abc" to any int value it doesnt work. For example if I say $21 it will think I want to use the subpattern 21 instead of using subpattern 2 then add a single "1". I tried ascii in the replace without any luck...

edit: well one way I csan bypass this problem in this scenario:

 $result = (preg_replace('/(.+href=)"(.+\..+)="/i', '$1"$2=1"', $string) );

I am still looking for a real fix to this problem tho.

[ZeratulsDomain]

bump?