usort function in a class

Question

I have a function which sorts data in a multidimensional array, as shown below:

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Answer 1

// If order_new is a NORMAL function 
usort($data, [$this, 'order_new']);      // self ref object, function name  

// If order_new is a STATIC function 
usort($data, [__CLASS__, 'order_new']);  // string, function name