basic pg lock question

I have a question about whether or not I need to do locking to a pg
table being accessed from a php application.

Let's say two users select rows from the table and display them in
their browser. User A clicks on row 1 to edit it. Since this is php,
so far it just selects the current values from the row. Eventually,
user A will submit his changes and update the row.

In the meantime, when User B looks at his web page, there will still
be an 'edit' link for row 1. I'm pretty sure that I don't want User B
to try to edit the row, but as far as I understand the default postgres
locking will not prevent this. When user A selects row 1, do I need to
manually lock the row to prevent another select?

I'm sorry if this is basic stuff!