Actually sscanf()_always_ returns an array if you specify less return variables than format specifiers. i may change this to return a scalar if only a single format specifier exists.
 Note that sscanf() is (almost) the complete functional equivalent of its "C" counterpart, so you can do the following to get the expected effect :
 sscanf("SN/2350001","SN/%d",&$serial)
The array return was a nicety for PHP.