Legal Question in Business Law in California

No, but not all contracts must be signed.

It depends on what you seek with the use of the use of a PO in lieu of a signature on a contract. If you want evidence of acceptance of the offer, a PO may indicate acceptance and, as such, would operate just as a signature. Assuming this is a sale of goods contract, the UCC has specific rules on such matters particularly applicable in contexts between commercial entities.

The answer is probably found in the Commercial Code's Division 2, Chapter 2, entitled "Form, Formation and Readjustment of Contract" and containing sections 2201 through 2210 of the Code. Note in particular:

Section 2204 which says, "A contract for sale of goods may be formed in any manner sufficient to show agreement, including conduct by both parties which recognizes the existence of such a contract. (etc.)"

Section 2206(1)(a) which says, "An offer to make a contract shall be construed as inviting acceptance in any manner and by any medium reasonable in the circumstances."

If the contract and the P.O. contain differing terms, section 2207 should be applied to determine whether, nevertheless, a contract was formed.

If you or your organization is regularly involved in contracting for the purchase or sale of goods, you should probably familiarize yourself with Division 2, Parts 1 through (at least) 3.