forth_stack.v2.php

<?php

function forth_calc($input) {
    $stack = array();

    foreach (explode(' ', $input) as $chunk) {
        if ($chunk === '+') {
            $stack[] = array_pop($stack) + array_pop($stack);
        } elseif ($chunk === '-') {
            $stack[] = array_pop($stack) - array_pop($stack);
        } elseif ($chunk === '*') {
            $stack[] = array_pop($stack) * array_pop($stack);
        } elseif ($chunk === '=') {
            $stack[] = array_pop($stack) === array_pop($stack);
        } elseif ($chunk === 'DUMP') {
            var_dump($stack);
        } elseif ($chunk === 'DUP') {
            $stack[] = end($stack);
        } else {
            $stack[] = $chunk;
        }
    }
    return $stack;
}

$input = '10 20 100 + - 20 * DUP 10 20 + *';
$t = microtime(1);
for ($i = 0; $i < 100000; $i++) {
    forth_calc($input);
}
echo 'Array: ', microtime(1) - $t, "\n";
print_r(forth_calc($input));

forth_stack.v3.php

<?php

function forth_calc($input) {
    $stack = array();
    $esi = 0;

    foreach (explode(' ', $input) as $chunk) {
        if ($chunk === '+') {
            $stack[--$esi - 1] = $stack[$esi] + $stack[$esi - 1];
        } elseif ($chunk === '-') {
            $stack[--$esi - 1] = $stack[$esi] - $stack[$esi - 1];
        } elseif ($chunk === '*') {
            $stack[--$esi - 1] = $stack[$esi] * $stack[$esi - 1];
        } elseif ($chunk === '=') {
            $stack[--$esi - 1] = $stack[$esi] === $stack[$esi - 1];
        } elseif ($chunk === 'DUMP') {
            var_dump($stack);
        } elseif ($chunk === 'DUP') {
            $stack[$esi] = $stack[$esi - 1];
            $esi++;
        } else {
            $stack[$esi++] = $chunk;
        }
    }
    return $stack;
}

$input = '10 20 100 + - 20 * DUP 10 20 + *';
$t = microtime(1);
for ($i = 0; $i < 100000; $i++) {
    forth_calc($input);
}
echo 'Array: ', microtime(1) - $t, "\n";
print_r(forth_calc($input));