sheet-letters-to-number.php

<?php

$converter = new Converter('ZZ');

echo $converter->getResult() . PHP_EOL;

class Converter
{
    private $letters;
    private $result = 0;

    public function __construct(string $letters)
    {
        $this->letters = str_split($letters);
        
        $this->setResult();
    }

    public function setResult()
    {
        $alphabet       = $this->getAlphabet(); 
        $alphabetLength = count($alphabet);
        $lettersLength  = count($this->letters);

        if(!$lettersLength) {
            return $this->result; // supposed to be 0
        }

        if($lettersLength === 1) {
            return $alphabet[$this->letters[0]] + 1;
        }

        for($i = 0; $i < count($this->letters); $i++)
        {
            $lengthAtCurrentPosition = $alphabet[$this->letters[$i]] + 1;
            $lettersLength -= 1;
    
            $this->result += (pow($alphabetLength, $lettersLength) * $lengthAtCurrentPosition);
        }
    }

    public function getAlphabet(): array
    {
        return array_flip(['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']);
    }

    public function getResult(): int
    {
        return $this->result;
    }
}

?>