Verify uniqueness of a new value entered when edited or added through phpGrid before saving
http://phpgrid.com/example/custom-data-validation/
The example only demonstrates client side validation. However, you can use jQuery ajax call to pass the new value to the server side and validate then send the response back. Here's an example on using ajax call
http://phpgrid.com/example/call-javascript-function-on-hyperlink-click/