get_missed_number

<?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));