The key genre will contain an integer between 0 and 147. You may use id3_get_genre_name() to convert it to a human readable string.
Since version 0.2 id3_get_tag() also supports ID3 tags of version 2.2, 2.3 and 2.4. To extract information from these tags, pass one of the constants ID3_V2_2, ID3_V2_3 or ID3_V2_4 as the second parameter.
([copyright] => Dirty Mac [originalArtist] => Dirty Mac [composer] => Marcus G?tze [artist] => Dirty Mac [title] => Little Big Man [album] => Demo-Tape [track] => 5/12 [genre] => (17)Rock [year] => 2001
)
ID3 v2.x tags can contain a lot more information about the MP3 file than ID3 v1.x tags.