<?php
// find missing element in the given array
$array = array(1,2,3,5);
// calculate length of given array
$length = count($array);
// actual length of given array
$actual_length = $length + 1;
// Calculate the sum of first n natural numbers as total sum = n*(n+1)/2
$total_sum = ( $actual_length * ( $actual_length + 1 ) ) / 2;
// missing number
$missing_number = $total_sum - array_sum($array);
echo $missing_number;
?>
OUTPUT : 4