plisp.php

<?php

function reduce($carry, $item) {
    $carry -= $item;
    return $carry;
}

function multi($carry, $item) {
    $carry *= $item;
    return $carry;
}

function div($carry, $item) {
    $carry /= $item;
    return $carry;
}

function myeval($tokens)
{
    $env = [];
    foreach ($tokens as $ex) {
        $t = $ex[0];
        switch (gettype($t)) {
            case "string":
                switch ($t) {
                    case "+":
                        return array_sum(array_slice($ex, 1));
                    case "-":
                        return array_reduce(array_slice($ex, 2), "reduce", $ex[1]);
                    case "*":
                        return array_reduce(array_slice($ex, 2), "multi", $ex[1]);
                    case "/":
                        return array_reduce(array_slice($ex, 2), "div", $ex[1]);
                }
            default:
                break;
        }
    }
}

function parse($str)
{
    $tokens = [];
    if ($str[0] !== "(") {
        return false;
    }
    // body
    $body = strstr(substr($str, 1), ")", true);
    $tokens []= explode(" ", $body);
    return $tokens;
}

function main($argc, $argv)
{
    if ( $argc !== 2 || $argv[1] === null) {
        fwrite(STDERR, "argument required");
        exit(1);
    }
    $tokens = parse($argv[1]);
    return myeval($tokens);
}

echo main($argc, $argv);