Note: this requires modifying the source code cls_datagrid.php which is only available to phpGrid commercial licenses.

phpGrid only validate data entry via client-side javascript. To display server-side error, modify the code to add errorfunc to grid.editRow in display_properties_main method so that users know that an server error occurred. The server will throw error during Ajax call similar to the following 

See the detail errorfunc usage here: http://www.trirand.com/jqgridwiki/doku.php?id=wiki%3ainline_editing#editrow excerpt: errorfunc: if defined, this function is called after the data is saved to the server. Parameters passed to this function are the rowid and the the response from the server request.