Verify uniqueness of a new value entered when edited or added through phpGrid before saving

You can use custom validation as shown in this example:
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/

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