<?php $numbers=array(1,4,7,13); function get_missed_number($sequence) { if(count($sequence)<2) { return false; } elseif(count($sequence)==2) { return ($sequence[1]-$sequence[0])/2; } else { $pDiff=$sequence[1]-$sequence[0]; for($i=1;$i<count($sequence);$i++) { if(($sequence[$i+1]-$sequence[$i])>$pDiff) { return $sequence[$i]+(($sequence[$i+1]-$sequence[$i])/2); }; }; }; } echo(get_missed_number($numbers));