I think it is much better to use the foreach like everyone else has suggested, but you could do it like the below:
$string = "1,4|2,64|3,0|4,18|";
$whichOne = "2";
echo "Starting String: $string <br>";
$pos = strpos($string, $whichOne);
//Accomodates for the number 2 and the comma
$valuepos = substr($string, $pos + 2);
$tempstring = explode("|", $valuepos);
$value = $tempstring[0]; //This will ow be 64
$newValue = $value + 18;
//Ensures you only replace the index of 2, not any other values of 64
$replaceValue = "|".$whichOne.",".$value;
$newValue = "|".$whichOne.",".$newValue;
$string = str_replace($replaceValue, $newValue, $string);
echo "Ending String: $string";
This results in:
Starting String: 1,4|2,64|3,0|4,18|
Ending String: 1,4|2,82|3,0|4,18|
You could run into issues if there is more than one index of 2... this will only work with the first instance of 2.
Hope this helps!