What are the rare varieties to look for on US/ Philippines coins?

Hussulo

I' going to help my father in law look through some of his coins but don't know what to look for?

I can remember talk of horned S's etc...

Thanks in advance.

IloiloKano

There are many varieties, but some are more "recognized" than others.

On the One Centavo bronzes (the series that I know the best) ...

1908 S/S and S/S/S
The S/S is commonly called the Horned S, since it has two "horns" portruding aboveThe S/S/S has horns above and below.
1909 S DD Reverse (Shield Side) with doubling on "NITED" most pronounced.1911 S/S1913 S/S1914 S Large S (Variety is disputed, though it is recognized by PCGS)1916 S Large S (Variety is disputed, though it is recognized by PCGS)1917/16 S1917/18 S1918 S Large S with and without top of 1 repunched
This variety is unquestioned, as the S is quite large and obvious
1930 M/M1931 M/M1933 M RPD1938 M RPD1944 S DD Reverse (Shield Side)
Several varieties with only one undisputed, which has a doubled scroll and doubled "AMERICA", most pronouced on the "M" and "C"

On the Five Centavos...

1918 S RPD1918 S/S1918 S MULE
This mule was accidentally produced using the reverse of a 1918 S 20 Centavos, which was very similar in size, but the date is much smaller.
1930 M RPD1934 M RP 11934 M DD Reverse and RP 1
Pronounced doubling on "MERIC"
1935 M RPD1945 S RP 5

On the Ten Centavos...

1907 S/S1908 S/S1908 S Inverted S1912 S/S1913 S/S1914 S Long Crossbar 41915 S/S1917 S RP 1 High Mint Mark1929 M RPD1944 D/D and D/D/D1944 D RPD1945 D/D1945 D DD Reverse
Two varieties with the Boldly Doubled variety (check the "RICA") much more expensive

On the Twenty Centavos...

1904 S RP 11904 S/S1905 S/S and RP 11908 S/S1908 S RP 81913 S/S1916 S Tilted 61928/7 M (Intentional) Mule (with a known mintage)
A 1927 Five Centavos Reverse die was altered (to 1928) to used for the 20 Centavos that year, and 100,000 were minted. The underdate is faint at best, but since there were no other 1928 Twenty Centavos minted that year, if you have one, then it is a Mule.
1929 M Triple RP 21929 M RP Final 91937 M RPD1944 D/S1944 D/D1945 D/D

On the Fifty Centavos...

1917 S Broken 71918 S Inverted S1944 S/S1945 S/S

On the One Peso crowns...

1905 S Straight Serif 11908 S/S1909 S/S and S/S/S

I believe there may be other (minor) varieties, but the above are the most recognized as true varieties. Hopefully, you will find this helpful.

Hussulo

Wow that's great!

Thank you for taking the time out to list them.