how to use googlecharts API: DataTable.toJSON()

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I want to use php to get data from mySQL database, then use googe charts to draw a chart. I used json_encode() to convert my query result table to JSON, but it does not work. It returned, the table has no columns. I noticed there is toJSON() function in Google charts API, but there is no detaile开发者_如何学JAVAd information. Anyone knows how to use this function? thanks