operadoresPublic.php

<?php

if(! function_exists('lastElement')) 
{
    function lastElement($lastElement)
    {
        if(!is_array($lastElement) || $lastElement == []){
            return  [];
        } else{
            return $lastElement[count($lastElement) - 1];
        }
    }
}

if(! function_exists('popArray')) 
{
    function popArray($popFunction)
    {
        if (is_array($popFunction)) {
            array_pop($popFunction);
            return $popFunction;
        } else {
            return [];
        }
    }
}

function operation(string $operator, array $values, $result = null)
{
    if($values === [])
    {
        return $result;
    }
    else
    {
        if ($result === null) 
        {
            $values = array_reverse($values);
            $result = lastElement($values);
            $values = popArray($values);
        }
            $sendEvalResult = eval('return '.$result.' '.$operator.' '.lastElement($values).';');
            return operation($operator, popArray($values), $sendEvalResult);
    }
}
echo(operation('.', [9, 2]));