Use array_count_values() to count the number of times each value appears in the array.
Use array_filter() to get the values that occur once in the array.
And finally use array_keys() to get the value from the array.

example code in PHP

<?php
	$marks = [12,13,13,14,12,15,15];
	$result = array_keys(array_filter(array_count_values($marks), function($v)
								{
								  return $v == 1;
								} ));
			print_r($result);
	?>