Fill Gap

<?php
$ar = array(16,17,19,19,20);
$action = false;
$new = array();
$temp = array();

foreach ($ar as $key => $val)
{
    if($key == 0 OR $ar[$key-1] == $val)
    {
        // if it's first value,
        // or current value is same as previous,
        // just add it as is
        $new[] = $val;
    }
    else{
        // else,
        // add all missing value including current one
        // to $new array
        for($x = $ar[$key - 1] + 1; $x <= $val; $x++)
        {
            $new[] = $x;
        }
    }
}
echo '<pre>';
print_r ( $new );
echo '</pre>';