XAMPP ships with a file ext/php_oci8.dll which contains the extension module. You have to enable that module (i.e. make php load the extension either at runtime or preferably at startup time).
First take a look at http://docs.php.net/oci8.requirements and check the requirements.
Then call
<?php echo 'ini: ', get_cfg_var('cfg_file_path');
It will tell you which php.ini you have to modify (depending on the version of xampp you're using there may be one ini for the apache module and a different ini for the cli version, or there may be only one file for both the module and the cli version).
In that ini file look for a line like*
;extension=php_oci8
Remove the semicolon (which made the line a comment) and restart the apache. Done (hopefully).
You might also be interested in the extensions
extension=php_pdo_oci
extension=php_pdo_oci8
which allow you to use PDO instead of the oci_connect(), oci_execute(), oci_* functions.
* In the build from php.net there are two slightly different extensions modules
;extension=php_oci8.dll ; Use with Oracle 10gR2 Instant Client
;extension=php_oci8_11g.dll ; Use with Oracle 11g Instant Client
XAMPP seems to ship with only one php_oci8.dll.