calc

echo "\n\n============Калькулятор============\n\n";

$calc = "2+2-8*10-7+3/2.5989=";
$znaki = array();
$separ = '!([0-9.]+)(\\D)!ui';
preg_match_all($separ, $calc, $znaki);
var_dump($znaki);
$actions = $znaki[2];
$znaki = $znaki[1];
for ($i = 0; $i < count($znaki); $i++){
    $znaki[$i] = floatval($znaki[$i]);
}
var_dump($znaki);
var_dump($actions);
$result = $znaki[0];
for ($i = 0; $i < count($znaki); $i++){
    if ($actions[$i] == "+"){
        $result = $result + $znaki[$i + 1];
    } elseif ($actions[$i] == "-"){
       $result = $result - $znaki[$i + 1];
    } elseif ($actions[$i] == "*"){
       $result = $result * $znaki[$i + 1];
    } elseif ($actions[$i] == "/"){
       $result = $result / $znaki[$i + 1];
    } elseif ($actions[$i] == "="){
       echo "\nИсходное выражение: {$calc}\nОтвет: {$result}";
    }
}