Selecting first number not in an array PHP

I have the following problem but I will simplify it here:

$starting = 0;
$arrayOfNumbersToSkip = [1, 3, 4, 5, 6];
$ending = 7;

Part 1: Give me the first number available? “Answer is 2”

Part 2: Give me the next number available? “Answer is 7”

This is a simplified version of a math problem im facing, im designing a dynamic approval system that modifies the entire program based on changeable rules.

Answer

You could use array_diff with a range from $starting+1 to $ending:

$availableNumbers = array_diff(range($starting+1, $ending), $arrayOfNumbersToSkip);
if (!count($availableNumbers)) {
    echo "no numbers availablen";
}
else {
    echo "first number available = " . array_shift($availableNumbers);
}
print_r($availableNumbers);

Output:

first number available = 2
Array
(
    [0] => 7
)

The output shows that there are still available numbers (you can keep using array_shift to get them) after taking 2 from the array.

Demo on 3v4l.org