is_a is deprecated in php

When we try to put E_STRICT in error reporttin we will get an error in xajax.inc.php for "is_a" in line 658 which is because of deprecation in 5.3 so replace the line which the line below
i.e we can use instanceof instead of is_a to check the object is of the specific class

    if ($sResponse instanceof xajaxResponse) {
}
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