Every will shall be in writing and shall be signed by the testator at the end thereof, subject to the following rules and exceptions:
(1) Words following signature.--The presence of any writing after the signature to a will, whether written before or after its execution, shall not invalidate that which precedes the signature.
(2) Signature by mark.--If the testator is unable to sign his name for any reason, a will to which he makes his mark and to which his name is subscribed before or after he makes his mark shall be as valid as though he had signed his name thereto: Provided, That he makes his mark in the presence of two witnesses who sign their names to the will in his presence.
(3) Signature by another.--If the testator is unable to sign his name or to make his mark for any reason, a will to which his name is subscribed in his presence and by his express direction shall be as valid as though he had signed his name thereto: Provided, That he declares the instrument to be his will in the presence of two witnesses who sign their names to it in his presence.