How to update specific key's value in an associative array in PHP?

Change your foreach to something like this, You are not assigning data back to your return variable $data after performing operation on that.

foreach($data as $key => $value)
{
  $data[$key]['transaction_date'] = date('d/m/Y',$value['transaction_date']);
}

Codepad DEMO.

This will work too!

foreach($data as &$value) {
  $value['transaction_date'] = date('d/m/Y', $value['transaction_date']);
}

Yay for alternatives!

PHP array_walk() function is specifically for altering array.

Try this:

array_walk ( $data, function (&$key) { 
    $key["transaction_date"] = date('d/m/Y',$key["transaction_date"]); 
});

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