Syntax problem on your arrays
$array1 = array(
'a' => 'value1',
'b' => 'value2',
'c' => 'value3',
);
$array2 = array(
'a' => 'value1',
'b' => 'value2',
'c' => 'value3',
);
$diff = array_diff($array1, $array2);
var_dump($diff);